xcb: Fix transient parent and "Qt::Window" flag

Don't set transient parent property when Qt::Window flag is set.
Delete transient parent property if a window doesn't have a transient parent.
Force setting standard window flags for Qt::Window only if there are no other
flags.

Amends 98c10a02



Task-number: QTBUG-52550
Change-Id: I68ee715b632487e9dd0e7ffbbfc0c2cdd0f0e151
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>