Introduce --exclude-titlebar flag for mouse bindings. (#2703)

This introduces the flag --exclude-titlebar for mouse bindings which
allows bindings like

    bindsym --whole-window --border --exclude-titlebar button3 focus

fixes #2347

include/data.h

@@ -298,6 +298,10 @@ struct Binding {
      * title bar (default). */
     bool whole_window;
 
+    /** If this is true for a mouse binding, the binding should only be
+     * executed if the button press was not on the titlebar. */
+    bool exclude_titlebar;
+
     /** Keycode to bind */
     uint32_t keycode;