Remember monitor mapping on close.
When the application is stopped, if the windows are in fullscreen, their
position on the client will be remembered.
This change uses the existing option 'monitor-mapping' in the settings
file to save the position and reuse it on next launch.
This implements part of the requirement from
NOTE: this feature is effective only with GTK >= 3.22
Reviewed-by: Daniel P. Berrangé <email@example.com>
Signed-off-by: Julien Ropé <firstname.lastname@example.org>