Settings: only write the prefix rows that were edited

Review findings on the prefix editor:

- OK without touching anything changed the file: every row was written
  back, so an explicit <prefix/>, which reads as an empty field and
  cancels the inheritance, was dropped and that folder started
  inheriting its parent's prefix again. A row is now written only when
  it was edited or no longer holds what the file has, and the two kinds
  of empty field look different: hasPrefix() tells an explicit
  <prefix/> from a missing one, which is what the field hint shows.
- load() copied a broken file aside before the user had chosen
  anything, so every "Corriger le fichier" left a .bak behind while the
  message said nothing had been modified. The copy is now made by
  save(), right before the file is replaced: repairing or cancelling
  leaves nothing behind, and a copy that cannot be made stops the write
  rather than destroying the only copy.
- The button followed the combo box only for "Parcourir...": put back
  on "Par defaut" it fell back to the previously saved path instead of
  the default one. The directory of the entry being displayed is now
  used, "Par defaut" being dataDir()/elements/.

sources/ElementsCollection/qetlabelsfile.cpp

@@ -207,16 +207,13 @@ bool QetLabelsFile::parse(QIODevice &device, QDomDocument &document, QString *re
 	- when the file does not exist yet, an empty \<labels\> document is
 	  kept in memory and will only be created on the first save()
 	- when the file exists but cannot be read, is not well formed or does
-	  not have \<labels\> as its root element, a copy of it is kept as a
-	  backup (@see backupPath()) and an empty document is used instead,
-	  so the broken file is replaced only if the caller saves. isBroken()
-	  and brokenReason() tell what is wrong with it, so the caller can
-	  offer repairing the file rather than silently discarding its
-	  content.
-	@return false when no collection directory was given, or when the file
-		is broken and could not be backed up : rebuilding it then would
-		destroy the only copy of it, so errorString() tells why and
-		nothing is loaded.
+	  not have \<labels\> as its root element, nothing is written : an
+	  empty document is used instead, and a copy of the unusable file is
+	  only made by save(), right before it is replaced. isBroken() and
+	  brokenReason() tell what is wrong with it, so the caller can offer
+	  repairing the file rather than silently discarding its content.
+	@return false only when no collection directory was given. It is
+		save() that reports a file it could not copy aside.
 */
 bool QetLabelsFile::load(const QString &collection_dir)
 {
@@ -263,19 +260,11 @@ bool QetLabelsFile::load(const QString &collection_dir)
 	if (well_formed) {
 		m_document = document;
 	} else {
-			//The file is unreadable or malformed : keep a copy of it, then
-			//work on an empty document. m_force_save makes sure the broken
-			//file is replaced as soon as the caller validates, even though
-			//the empty document we start from serializes just fine.
-		m_backup_path = backupBrokenFile();
-		if (m_backup_path.isEmpty()) {
-				//Without a backup, rebuilding would destroy the only copy
-				//of the file : stop here instead of letting the caller
-				//overwrite it.
-			m_error = tr("Le fichier %1 est endommagé (%2) mais n'a pas pu être sauvegardé :\nrien n'a été modifié.")
-					.arg(m_file_path, m_broken_reason);
-			return false;
-		}
+			//The file is unreadable or malformed : leave it alone and work
+			//on an empty document. m_force_save makes sure the file is
+			//replaced as soon as the caller validates, even though the
+			//empty document we start from serializes just fine - and it is
+			//also what tells save() to copy the file aside first.
 		m_broken = true;
 		createEmptyDocument();
 		m_force_save = true;
@@ -356,6 +345,27 @@ QString QetLabelsFile::prefix(const QStringList &relative_path) const
 	return value.isNull() ? QString() : value;
 }
 
+/**
+	@brief QetLabelsFile::hasPrefix
+	@return true when @a relative_path owns a \<prefix\> child of its own,
+		even an empty one. prefix() alone cannot tell that apart from a
+		category without any \<prefix\> : both give it no value, while an
+		empty \<prefix/\> cancels the inheritance where a category
+		without one inherits.
+*/
+bool QetLabelsFile::hasPrefix(const QStringList &relative_path) const
+{
+	QDomElement node = m_document.documentElement();
+	for (const QString &name : relative_path) {
+		node = directChildCategory(node, name);
+		if (node.isNull()) {
+			return false;
+		}
+	}
+
+	return !node.firstChildElement(QStringLiteral("prefix")).isNull();
+}
+
 /**
 	@brief QetLabelsFile::directChildCategory
 	@return the \<category\> child of @a parent named @a name, or a null
@@ -617,6 +627,20 @@ bool QetLabelsFile::save()
 		return false;
 	}
 
+	if (m_force_save && m_file_exists) {
+			//The file on disk could not be read : copy it aside before it
+			//is replaced. Doing it here, and not when it is loaded, means
+			//no copy is left behind when the user chooses to repair the
+			//file instead of rebuilding it - or simply changes their mind
+			//and cancels.
+		m_backup_path = backupBrokenFile();
+		if (m_backup_path.isEmpty()) {
+			m_error = tr("Le fichier %1 n'a pas pu être copié à côté avant d'être remplacé :\nrien n'a été modifié.")
+					.arg(m_file_path);
+			return false;
+		}
+	}
+
 	QSaveFile file(m_file_path);
 	if (!file.open(QIODevice::WriteOnly | QIODevice::Text)) {
 		m_error = file.errorString();