Avoid QXcbEventReader crash (bnc#879295).

The crash happened because Qt was unloaded before QApplication was
destroyed. The XCB thread was still running and an X message was handled
by missing code.

QApplication::deleteLater did not work, delete it directly.

This introduces
<libqt-warning> QThreadStorage: Thread 0x620c80 exited after QThreadStorage 2 destroyed
<libqt-warning> QThreadStorage: Thread 0x620c80 exited after QThreadStorage 1 destroyed
so that will still need some debugging.
This is a quick fix to unblock a release.
version + changelog
When using libyui threads, destroy qApp in the right one.
Merge pull request #39 from mvidner/xcb-crash

Avoid QXcbEventReader crash during library shutdown (bnc#879295).