Add set_modal ond unset_modal to the gtk_surface interface. When a
surface is modal, the compositor can treat it differently from non-modal
dialogs, for example attach it to the parent window if any. There is
currently no changes to input device focus; it is up to the client to
ignore events to the parent surface that is wanted.
This bumps the gtk_shell version to 2.
https://bugzilla.gnome.org/show_bug.cgi?id=745720
This is also something that we did upstream. Since we want to
introduce an explicit "xdg_transient" window type for tooltips
and popovers, and since "transient_for" is a confusing dumb
80s term lifted from the ICCCM spec, just rename it.
This was changed upstream a little while ago for C++ compatibility.
It's also the more common term for the operation: you close a window,
you don't delete one. In fact, a delete event might seem like it
would be about resource management instead.
