libweston/desktop, desktop-shell: Add getters for pending state

This introduces a few getters to retrieve the pending state from
libweston-desktop, now just libweston, and makes use of it,
specifically get_pending_maximized to avoid sending invalid
dimensions to the client in the particular use case
set_maximized/unset_fullscreen.

These pending state getters are useful to query/poke a not-applied
yet state, and could be useful where we don't have a buffer attached
where the client might be set-up as maximized, but internally libweston
hasn't yet applied that pending state.

Fixes #645

Suggested-by: Morgane Glidic <sardemff7+git@sardemff7.net>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>