Fix excess enter events when showing modal dialog on button press

Fix excess enter events when showing modal dialog on button press

QApplicationPrivate::leaveAfterRelease was not properly cleared when
mouse event handling was interrupted by a modal dialog, which caused
every mouse move over the modal dialog to trigger enter event to the
widget under cursor.

Fixed by clearing QApplicationPrivate::leaveAfterRelease if mouse event
without any buttons pressed is handled.

Task-number: QTBUG-27643
Change-Id: I4f31daa656bc643c88e5338282a671ae2077e255
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>