Refresh MonitorArrangement on configuration change

Refresh MonitorArrangement on configuration change

MonitorArrangement (in the options dialog) never changes when the system
monitor configuration changes. Therefore, the user can get into a state
where the reality doesn't match what is shown (when a monitor is
added/removed, resolution or position changed etc).

All these changes triggers an event in FLTK
(FL_SCREEN_CONFIGURATION_CHANGED). This commit adds an event handler in
MonitorArrangement and refreshes the widget whenever that event occurs.

Because Fl_Handler does not have a void*-argument (and we must be able
to access the widget from our handler callback) a static set of
instances have been added, which all will receive the events.