Report salvaged and unreadable font descriptions after opening a project

Until now a font description that could not be parsed only produced
console warnings most users never see, so nobody learned that their
texts silently lost their formatting (see the reports in issue #553).

Count in QETUtils::fontFromString() how many descriptions were salvaged
from a foreign or corrupt format and how many stayed unreadable, and
show a message box after opening a project when either happened:
salvaged descriptions are rewritten in the stable format on the next
save, unreadable ones fall back to the default font. Projects without
font issues open exactly as before, and non-interactive opens only log
the counters.

Verified with a Qt 5.15 build on a project carrying 52 19-field and
one 21-field description: the dialog reports 53 restored descriptions;
the same file on a Qt 6.11 build (which parses those formats natively)
shows no dialog.

See issue #553.

sources/utils/qetutils.cpp

@@ -153,6 +153,12 @@ void QETUtils::pixelSizedFont(QFont &font)
 
 namespace
 {
+	/* Counters for fontFromString(), reset per project load so the editor
+	 * can report how many stored font descriptions needed salvaging or were
+	 * unreadable. Font parsing only happens on the main thread. */
+	int salvaged_font_count = 0;
+	int unreadable_font_count = 0;
+
 	/**
 	 * Legacy (Qt 5) weight <- OpenType weight, closest match,
 	 * same table Qt uses when parsing a 10/11 field string.
@@ -235,6 +241,10 @@ QString QETUtils::fontToString(const QFont &font)
  */
 bool QETUtils::fontFromString(QFont &font, const QString &description)
 {
+	if (description.trimmed().isEmpty()) {
+		return false;
+	}
+
 	QFont parsed(font);
 	if (parsed.fromString(description)) {
 		font = parsed;
@@ -271,8 +281,10 @@ bool QETUtils::fontFromString(QFont &font, const QString &description)
 
 		if (parsed.fromString(legacy)) {
 			font = parsed;
+			++salvaged_font_count;
 			return true;
 		}
+		++unreadable_font_count;
 		return false;
 	}
 
@@ -284,7 +296,41 @@ bool QETUtils::fontFromString(QFont &font, const QString &description)
 	if (count > 11
 		&& parsed.fromString(QStringList(l.mid(0, 11)).join(comma))) {
 		font = parsed;
+		++salvaged_font_count;
 		return true;
 	}
+	++unreadable_font_count;
 	return false;
 }
+
+/**
+ * @brief QETUtils::resetFontRestorationCounters
+ * Reset the counters incremented by fontFromString(), to be called before
+ * loading a project so the numbers reported afterwards are per-project.
+ */
+void QETUtils::resetFontRestorationCounters()
+{
+	salvaged_font_count = 0;
+	unreadable_font_count = 0;
+}
+
+/**
+ * @brief QETUtils::salvagedFontCount
+ * @return How many font descriptions fontFromString() restored from a
+ * foreign or corrupt format since the counters were last reset. Such
+ * descriptions are rewritten in the stable format on the next save.
+ */
+int QETUtils::salvagedFontCount()
+{
+	return salvaged_font_count;
+}
+
+/**
+ * @brief QETUtils::unreadableFontCount
+ * @return How many font descriptions fontFromString() could not restore at
+ * all since the counters were last reset (the caller's default font applies).
+ */
+int QETUtils::unreadableFontCount()
+{
+	return unreadable_font_count;
+}

sources/utils/qetutils.h

@@ -34,6 +34,9 @@ namespace QETUtils
     void pixelSizedFont (QFont &font);
     QString fontToString (const QFont &font);
     bool fontFromString (QFont &font, const QString &description);
+    void resetFontRestorationCounters ();
+    int salvagedFontCount ();
+    int unreadableFontCount ();
 
 	bool sortBeginIntString(const QString &str_a, const QString &str_b);