Allow a mouse binding to hide a window, and add a default keybinding for CMS-M3,

so it's hard to press by accident, but there if you need it.

requested (in a way) and tested by johan and todd.

mousefunc.c

@@ -48,6 +48,12 @@ mousefunc_window_lower(struct client_ctx *cc, void *arg)
 	client_lower(cc);
 }
 
+void
+mousefunc_window_hide(struct client_ctx *cc, void *arg)
+{
+	client_hide(cc);
+}
+
 void
 mousefunc_menu_group(struct client_ctx *cc, void *arg)
 {