Use XI2 event detail to determine changed mouse button

The button state part of the XI2 events appears to be badly constructed
on some devices and platforms. Even where supported the 'detail' field
of the XI2 events is what we should be reading since it indicates the
button the event refers to and not just the state of all buttons.

Task-number: QTBUG-38169
Change-Id: Iedb7971194b3c27448b72c285a54100c511c17e4
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>