Update MonitorArrangement widget after change

Update MonitorArrangement widget after change

If the options dialog was open when a screen configuration happened the
widget could get out of sync from the settings. A scenario when this
happened was:

 1) 3 monitors, fullscreen selected on the two right-most screens
 2) disconnect the left-most screen (the one not selected)

In this case, using GNOME, vncviewer would appear in fullscreen on the
right of the two remaining monitors, but the widget would show both
monitors selected. The reason was that the MonitorArragement index
doesn't work the same way as FLTK's screen index.

It's debatable how vncviewer should behave here, but the GUI should at
least match the actual setting.