Add new cycling option

When using Xinerama, make it possible only to cycle between those
windows that are on the currently active head - AFAICT this means
windows that have the majority of their area on the same head as
the pointer.

Patch complete with WPrefs integration, alas this being quite
unusable - for a lack of a better idea, I tucked the option on the
expert panel, making the item list too tall - making it scrollable
would be a nice addition.

The default for the new option (CycleActiveHeadOnly) should be NO,
that is, no change in default behaviour.

src/defaults.c

@@ -655,7 +655,9 @@ WDefaultEntry optionList[] = {
 	{"SelectCursor", "(builtin, cross)", (void *)WCUR_SELECT,
 	 NULL, getCursor, setCursor},
 	{"DialogHistoryLines", "500", NULL,
-	 &wPreferences.history_lines, getInt, NULL}
+	 &wPreferences.history_lines, getInt, NULL},
+	{"CycleActiveHeadOnly", "NO", NULL,
+	 &wPreferences.cycle_active_head_only, getBool, NULL}
 };
 
 #if 0