QGuiApplication: clear font database on cleanup

Change-Id: If196c5e93b7ed68c6a0f6be7414f9efc8195f168
Task-number: QTBUG-40865
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>

src/gui/kernel/qguiapplication.cpp

@@ -588,6 +588,7 @@ QGuiApplication::~QGuiApplication()
 #endif //QT_NO_SESSIONMANAGER
 
     clearPalette();
+    QFontDatabase::removeAllApplicationFonts();
 
 #ifndef QT_NO_CURSOR
     d->cursor_list.clear();