Fix hang-on-exit issue

This has been the most exhausting debugging session ever (took me more
than two days), and of course ended up in a one-liner fix...

Main problem:

Splash screen widget calling QCA::processEvents outside of an event loop

What causes the hang on exit:

Ultimately, QCA is never quit because some object never called
QCoreApplicationPrivate::deref. The funny thing is figuring out *which*
object is causing this. After scripting GDB to print backtraces after
each encounter of both QCoreApplicationPrivate::{ref,deref}(), I
eventually figured out that some KIO::ListJob instance is never calling the 'deref'
method.

After debugging further I noticed that this instance was *never* actually deleted,
thus not calling 'deref'. After another exhausting session I figured out
that the DeferredDelete event KJob sends when the job finished was never processed
by the event loop. This causes the ListJob to stay alive and block QCA
from quitting the event loop.

After checking bug reports and (test) code in
qtbase.git it finally appeared to me that the actual problem resided in our
splash screen widget calling QCA::processEvents *outside* the event
loop. When doing this, deferred deletes are not processed until
explicitly calling QCA::sendPostedEvents. Fix this by calling
QCA::sendPostedEvents each time we do QCA::processEvents.

Lesson learned: Should've bisect'ed this much earlier, to find out that the
port of the splash screen to QQ2 caused this behavior. Otoh, I thought
that the hang-on-exit has always been there on Frameworks...