Store popup parents separately from transient-for
parents, since these are separate concepts with
different behaviors. And we need the parent in
the frontend, so we can use it in the fallback
move-to-rect implementation.
We don't need the complicated wrapper system anymore,
since client-side windows are gone. This commit moves
all the vfuncs to GtkSurfaceClass, and changes the
backends to just derive their surface implementation
from GdkSurface.
