Introduce a cached boolean for changes to the mark of a window.

This is necessary to correctly redraw window decorations when the mark of
a window is added or removed.

include/data.h

@@ -543,6 +543,8 @@ struct Con {
 
     /* user-definable mark to jump to this container later */
     char *mark;
+    /* cached to decide whether a redraw is needed */
+    bool mark_changed;
 
     double percent;