xcb: Properly process enter/leave events

Ignore enter/leave events when there is a window under mouse button.
Unset window under mouse button if other window is grabbed.
Smarter ignoring (un)grab ancestor enter/leave event.
Ignore ungrab inferior leave event.

Amends: b9f76db3



Task-number: QTBUG-46576
Task-number: QTBUG-51573
Task-number: QTBUG-52332
Task-number: QTBUG-52488
Change-Id: I8d926309aa60bb8929728691c31ecf93d1e299ad
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>