Revert "Implement automatic mouse grabbing on mouse button press."

Revert "Implement automatic mouse grabbing on mouse button press."

This reverts commit 6b5bbc531b30d8ece25425e39843c6ae1af1d045.

Autograbbing mouse shouldn't be done in crossplatform code, as
X11 does this automatically. Windows needs platform specific
solution.

Task-number: QTBUG-26962
Task-number: QTBUG-27039
Task-number: QTBUG-23699

Change-Id: I911df92c4a34deb50b729f50681497046657948b
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>