xcb: Properly unset mousePressWindow()

In some cases the mouse release event won't arrive, i.e. when window is
minimized on button press. Check for mouse buttons state on mouse move
event and properly unset the mousePressWindow to avoid blocking
enter/leave events in this case.

Amends: c511466d



Change-Id: I543a75104f528df1bf644bace13f78a6af017455
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>