Auto adjust resolutions for mirror mode

Submitted by Evgen

Assigned to Xfce Bug Triage

Link to original bug (#15710)

Description

Problem: You connect notebook with lower resolution display (1360x768) to different screen / projector with higher resolution (1920x1080) and set "mirorred mode". Both image sources have their maximum resolution. As result image "bottom" of the notebook display will be cut. It can be fixed by setting the same maximum resolution for both sources (1360x678). It's a long standing issue (pre-GTK3 port) which spoils user experience. Luckily now we have automatic monitor profiles but other desktop envirornment (MATE) handles situation perfectly by default.