Resolve relative compiled-in data paths from the binary, not the CWD

commonElementsDir(), commonTitleBlockTemplatesDir() and languagesPath()
return the compile-time path verbatim when it is not marked
*_RELATIVE_TO_BINARY_PATH. On Windows those paths are relative
("./elements/", "./titleblocks/", "./lang/"), so they resolve against the
process working directory.

That only holds when QET is started from its own installation folder.
Opening a document from a file manager sets the working directory to the
document's folder, and the data is then looked for next to the user's
file. The shortcuts hide this by passing --common-elements-dir,
--common-tbt-dir and --lang-dir explicitly; anything that launches the
binary without them does not (see #554).

Add resolveConfiguredDataPath(): absolute paths are returned unchanged,
and a relative one is tried against the working directory first (so any
setup relying on the old behaviour keeps working), then next to the
executable, then in its parent -- the layout used by the Windows
packaging, where the binary sits in bin/ with the data beside it. This is
the same fallback the no-compile-option branch already performs for
issue #86, which was unreachable whenever the compile option is set.

The *_RELATIVE_TO_BINARY_PATH defines are left alone; they are only set
for macOS in qelectrotech.pro, and the CMake guard that would set them
tests a variable that is never defined.

sources/qetapp.cpp

@@ -539,6 +539,48 @@ TitleBlockTemplatesCollection *QETApp::titleBlockTemplatesCollection(
 	return(nullptr);
 }
 
+/**
+	@brief resolveConfiguredDataPath
+	Resolve a data directory baked in at compile time.
+
+	An absolute path is returned unchanged. A relative one used to be
+	interpreted against the process working directory, which is only correct
+	when QET is started from its own installation folder: opening a document
+	from a file manager sets the working directory to the document's folder,
+	so the data was not found there. Resolve it against the executable
+	instead, trying the folder next to the binary and then its parent -- some
+	packagings put the binary in a "bin" subfolder with the data beside it
+	(see issue #86).
+
+	The working-directory interpretation is still attempted first, so any
+	setup that relied on it keeps working.
+
+	\~French Resout un dossier de donnees fixe a la compilation.
+	@param configured : the compile-time path
+	@return an existing directory if one is found, @a configured otherwise
+*/
+static QString resolveConfiguredDataPath(const QString &configured)
+{
+	if (configured.isEmpty() || QDir::isAbsolutePath(configured)) {
+		return(configured);
+	}
+	if (QDir(configured).exists()) {
+		return(configured);
+	}
+
+	const QString bin_dir = QCoreApplication::applicationDirPath();
+	const QStringList candidates = {
+		QDir::cleanPath(bin_dir + "/" + configured) + "/",
+		QDir::cleanPath(bin_dir + "/../" + configured) + "/"
+	};
+	for (const QString &candidate : candidates) {
+		if (QDir(candidate).exists()) {
+			return(candidate);
+		}
+	}
+	return(configured);
+}
+
 /**
 	@brief QETApp::commonElementsDir
 	@return the dir path of the common elements collection.
@@ -586,7 +628,8 @@ QString QETApp::commonElementsDir()
 		/* the compilation option represents a classic absolute
 		 *  or relative path
 		 */
-		m_common_element_dir = QUOTE(QET_COMMON_COLLECTION_PATH);
+		m_common_element_dir =
+				resolveConfiguredDataPath(QUOTE(QET_COMMON_COLLECTION_PATH));
 		return m_common_element_dir;
 #else
 		/* the compilation option represents a path
@@ -754,7 +797,7 @@ QString QETApp::commonTitleBlockTemplatesDir()
 	#ifndef QET_COMMON_COLLECTION_PATH_RELATIVE_TO_BINARY_PATH
 		// the compile-time option represents a usual path
 		// (be it absolute or relative)
-		return(QUOTE(QET_COMMON_TBT_PATH));
+		return(resolveConfiguredDataPath(QUOTE(QET_COMMON_TBT_PATH)));
 	#else
 		/* the compile-time option represents a path relative
 		 * to the directory that contains the executable binary
@@ -1261,7 +1304,7 @@ QString QETApp::languagesPath()
 		 * l'option de compilation represente
 		 *  un chemin absolu ou relatif classique
 		 */
-		return(QUOTE(QET_LANG_PATH));
+		return(resolveConfiguredDataPath(QUOTE(QET_LANG_PATH)));
 	#else
 		/* the compilation option represents a path relative
 		 *  to the folder containing the executable binary