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];