A transient window should always be visible on the current screen.
This change ensures that a transient is always placed in a way that
the transient window is visible on the screen ignoring the transient
offset hint if it has to be.
Unfortunately QtWayland doesn't set the transient hint correctly:
a sub menu is a transient to the main window and not to the parent
menu resulting in quite off positioned menus, see:
https://bugreports.qt.io/browse/QTBUG-51640
Need to add client pos to the transient's position. The offset is
relative to the parent surface, but the client doesn't know about the
size of the decoration, thus KWin needs to add it.
Test creates windows and transients for it and verifies the position.
For decorated windows the position is currently broken as it does not
consider the adjusted client position.
