Position windows with transient parents properly

Position windows with transient parents properly

Both the transient position calculation and the global mouse
position had to be fixed.

Combo box contents, menus, etc. will now all appear at the
correct position. With qwindow-compositor they will
keep the proper position even when the parent is moved
via the decoration. Not sure how this could be achieved
with weston as there is no notification when the window
is dragged with Super+Mouse. Nevertheless the initial
position will be correct.

Change-Id: I66eca9faa930af71b9b540018d5057af7f5f22e5
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>