If a client creates an xdg_popup given a parent that is a xdg_popup that
is not the most top one in the grab chain, send the
not_the_topmost_popup error.
Also fail a client who destroys a popup that is not the top most one.
https://bugzilla.gnome.org/show_bug.cgi?id=744452
We'll want to expose popup logic outside of meta-wayland-pointer.c and
one day we'll also probably want to add touch support for popups, so
lets move it to its own file. There are no significant semantical
changes, only refactoring.
https://bugzilla.gnome.org/show_bug.cgi?id=744452