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.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);