xdg-shell: Add support for explicit popup repositioning

This commit adds protocol additions making it possible to request that a
popup should be repositioned according to a new xdg_positioner object.

Explicit popup moving is done using a new request on xdg_popup:
xdg_popup.reposition. What it does is change the parameters used for
positioning a popup by providing a new xdg_positioner object. This
request is coupled with a new event; xdg_popup.repositioned, sent
together with the configure events (xdg_popup.configure and
xdg_surface.configure) to notify about the completion of the reposition
request. The reposition request also takes a token that is later passed
via the repositioned event; this is done so that a client may determine
for which reposition request the compositor has sent configure events.

Synchronization between surfaces to avoid state application race
condition are deliberately left out, and should be handled by an
external protocol.

To brief the compositor of the future dimension of the parent that the
compositor should position the popup against, a
xdg_positioner.set_parent_size request is added.

Lastly, a request to couple a xdg_positioner object with a parent
configure event is added (xdg_positioner.set_parent_configure) in
order for a compositor to pair a popup reposition request with a pending
configure event, and it's resulting window geometry. This is necessary
to, for example, properly constrain a popup given a future parent state.
An example of when this may be necessary is an interactive resize where
both the toplevel position and the relative popup position changes.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
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