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/WindowMaker.h

@@ -413,6 +413,7 @@ typedef struct WPreferences {
     char workspace_border_position;     /* Where to leave a workspace border */
     char single_click;                  /* single click to lauch applications */
     int history_lines;                  /* history of "Run..." dialog */
+    char cycle_active_head_only;        /* Cycle only windows on the active head */
 
     RImage *swtileImage;
     RImage *swbackImage[9];

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

src/switchpanel.c

@@ -80,6 +80,11 @@ static int canReceiveFocus(WWindow * wwin)
 {
 	if (wwin->frame->workspace != wwin->screen_ptr->current_workspace)
 		return 0;
+
+	if (wPreferences.cycle_active_head_only &&
+	    wGetHeadForWindow(wwin) != wGetHeadForPointerLocation(wwin->screen_ptr))
+		return 0;
+
 	if (!wwin->flags.mapped) {
 		if (!wwin->flags.shaded && !wwin->flags.miniaturized && !wwin->flags.hidden)
 			return 0;