EdgeScroll: a few improvements

This addresses some edgescroll improvements and simplifies the logic
between how panframes are handled between being in global and per-monitor
mode.

Now, monitor edges which border one another don't have a panframe added
for global mode (only the outside edges are).  With the same logic,
per-screen panframes are drawn for all edges.

This also helps to reduce problems where some screens have different
resolutions, and so now dead spave between monitors is handled much
better.

Fixes #523

fvwm/events.c

@@ -4089,6 +4089,7 @@ void dispatch_event(XEvent *e)
 			XRRUpdateConfiguration(e);
 			monitor_update_ewmh();
 			monitor_emit_broadcast();
+			initPanFrames();
 			break;
 		}
 	}