Fix crash at shutdown.

The context pointer was dereferenced after the context object was deleted. This is fixed by storing a pointer to the MainWindow in QTFullScreen, so context does not have to be dereferenced.

The logic in QTFullScreen::eventFilter is also simplified a bit.

I found this bug after I noticed that GoldenCheetah started with the crash recovery dialog every time. I compiled with  address sanitizer (a gcc 4.8 feature copied from clang. See http://blog.qt.digia.com/blog/2013/04/17/using-gccs-4-8-0-address-sanitizer-with-qt/) and found where it crashed.