Regression: Hiding QWidget exits event loop

Change a512e210ac5b032c5fc2edf1ddf72e5a414485fd / http://codereview.qt-project.org/#change,10408 added the reference counting / quit lock feature to the event loop handling. Unfortunately the change introduced a regression that's affecting us in WebKit:

Hiding a QWidget causes QWindow::setVisible to be called, which dereferences the application and may cause the event loop to terminate. The attached simple test case reproduces the issue.

Compiled with Qt 4 pressing the button merely hides it but the event loop is not terminate. In Qt 5 after the change the event loop is terminated.