xwm: Preserve unmaximized window size at map time

This fixes a problem that can be seen with Firefox running under xwayland.
- Start it (or another client that remembers its size and maximized state),
- resize it
- maximize it
- exit it
- restart it
- unmaximize it

The size will be changed to a default size and not the previous unmaximized
size.

To fix this, save the unmaximized height and width like we do for the
fullscreen case.

Co-authored-by: Steve Pronovost <spronovo@microsoft.com>
Co-authored-by: Brenton DeGeer <brdegeer@microsoft.com>
Signed-off-by: Hideyuki Nagase <hideyukn@microsoft.com>
Signed-off-by: Steve Pronovost <spronovo@microsoft.com>
Signed-off-by: Brenton DeGeer <brdegeer@microsoft.com>
Signed-off-by: Derek Foreman <derek.foreman@collabora.com>