QFontDatabase: Clear font cache before emitting fontDatabaseChanged

Matches the ordering in QFontDatabasePrivate::invalidate() and ensures
that when the fontDatabaseChanged signal is processed by clients the
cache is ready to be filled again.

Change-Id: I9d09060284c6090ae2748147f6ee75b9f9b173c5
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/text/qfontdatabase.cpp

@@ -2188,12 +2188,12 @@ int QFontDatabasePrivate::addAppFont(const QByteArray &fontData, const QString &
 
     applicationFonts[i] = font;
 
-    emit qApp->fontDatabaseChanged();
-
     // The font cache may have cached lookups for the font that was now
     // loaded, so it has to be flushed.
     QFontCache::instance()->clear();
 
+    emit qApp->fontDatabaseChanged();
+
     return i;
 }