QPA: Clean shutdown (no caches, no reachable memory) of Qt to easily spot memory leaks

Currently starting a simple application and closing it (e.g. the syntaxhighlighter example) will show memory leaks/reachable memory in the valgrind analysis. This is due some bugs but also due different caches of Qt (font, pixmap).

It would be good for plugin creator and application designers to have Qt shutdown without any leak/still reachable memory.