Add support for transient to ShellSurface(Interface)

On client side a setTransient method is added which wraps the semantic
of wl_shell_surface_set_transient.

On server side both set_transient and set_popup are implemented, though
for popup only the transient part is implemented. In particular the grab
is not yet handled and also no popup done is provided.

For the transient on server side the flags are ignored. Main reason is
that Qt does not use the flag, so testing whether it works is tricky
(needs a test application).

REVIEW: 125223