Touch devices: ungrab handled incorrectly

We've the case that certain parts of our UI work unreliable when it comes to touch events. Sometimes, events are propagated to other, unrelated, elements.

I'm unable to produce a simple example for that, but we were able to reproduce this with the real app on iOS with the debugger connected and log output all around the Qt eventing code. The attached patch fixes these issues for us. Please check if it makes sense.

Will place it in gerrit (after I remember how that worked).