xcb: Ignore Mouse Enter:ungrab, Leave:grab events.

These are "special" XCB_NOTIFY_MODE_(UN)GRAB
Enter/Leave events and we do not have handlers for
them in Qt, so lets just ignore events with this mode.

Patch fixes Qt+ArchLinux+Awesome WM issues, where Qt
applications were receiving Enter/Leave events for
mouse clicks in the application window.

This patch does not affect "normal" XCB_NOTIFY_MODE_NORMAL
Enter/Leave event handling.

Task-number: QTBUG-45818
Change-Id: Ib70fdd9ed9200364a9753904f8e63d1ed9e2072f
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>