Feature: implement mouse bindings

A configured mouse binding (for example `bindsym button3 kill`) runs
its command when the mouse button is pressed over parts of a container.

If the binding has no modifer, it will only run when the button is
clicked on the window titlebar.

Otherwise if the binding has a modifier, it will run over the titlebar
or any part of the contained window.

fixes #558

src/key_press.c

@@ -30,7 +30,7 @@ void handle_key_press(xcb_key_press_event_t *event) {
     if (bind == NULL)
         return;
 
-    CommandResult *result = run_binding(bind);
+    CommandResult *result = run_binding(bind, NULL);
 
     if (result->needs_tree_render)
         tree_render();