xdg: support xdg-shell v3 with popup repositioning

xdg: support xdg-shell v3 with popup repositioning

See https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/3514
which added support on the wlroots side.

We now re-run popup positioning (for both xdg-shell and layer-shell
popups) when the "reposition" event is received. This allows popups that
change size (such as qmpanel's applications menu) to be positioned
correctly.

xdg-shell v3 also gives the compositor some additional "hints" for popup
positioning (reactive, parent_size, and parent_configure_serial) which
are available but we don't make use of currently.