Add new "KeepDockOnPrimaryHead" option

Once implemented, this will keep the dock on the primary head (when
there are multiple heads) when YES.  The default value is NO, the
current behavior, i.e., treat all monitors together as one large screen.

src/WindowMaker.h

@@ -405,6 +405,7 @@ extern struct WPreferences {
 	char ignore_gtk_decoration_hints;
 
 	char dont_blink;                   /* do not blink icon selection */
+	char keep_dock_on_primary_head;    /* keep dock on primary head */
 
 	/* Appearance options */
 	char new_style;                    /* Use newstyle buttons */

src/defaults.c

@@ -521,6 +521,8 @@ WDefaultEntry optionList[] = {
 	    &wPreferences.minipreview_size, getInt, NULL, NULL, NULL},
 	{"IgnoreGtkHints", "NO", NULL,
 	    &wPreferences.ignore_gtk_decoration_hints, getBool, NULL, NULL, NULL},
+	{"KeepDockOnPrimaryHead", "NO", NULL,
+	    &wPreferences.keep_dock_on_primary_head, getBool, NULL, NULL, NULL},
 
 	/* style options */