MonitorManager: Take into account all monitors when setting the fb size

[endlessm/eos-shell#2785]
monitor-xrandr: move variable locally

[endlessm/eos-shell#2785]
monitor-xrandr: always ungrab display

[endlessm/eos-shell#2785]
Merge pull request #12 from endlessm/shell/2785

Shell/2785