Jonas Ådahl authored Feb 14, 2020 and Carlos Garnacho committed Feb 29, 2020

This commits adds support on the MetaWindow and constraints engine side
for asynchronously repositioning a window with a placement rule, either
due to environmental changes (e.g. parent moved) or explicitly done so
via `meta_window_update_placement_rule()`.

This is so far unused, as placement rules where this functionality is
triggered are not yet constructed by the xdg-shell implementation, and
no users of `meta_window_update_placement_rule()` exists yet.

To summarize, it works by making it possible to produce placement rules
with the parent rectangle a window should be placed against, while
creating a pending configuration that is not applied until acknowledged
by the client using the xdg-shell configure/ack_configure mechanisms.

An "temporary" constrain result is added to deal with situations
where the client window *must* move immediately even though it has not yet
acknowledged a new configuration that was sent. This happens for example
when the parent wi...