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

include/bindings.h

@@ -61,9 +61,10 @@ void switch_mode(const char *new_mode);
 void check_for_duplicate_bindings(struct context *context);
 
 /**
- * Runs the given binding and handles parse errors. Returns a CommandResult for
- * running the binding's command. Caller should render tree if
- * needs_tree_render is true. Free with command_result_free().
+ * Runs the given binding and handles parse errors. If con is passed, it will
+ * execute the command binding with that container selected by criteria.
+ * Returns a CommandResult for running the binding's command. Caller should
+ * render tree if needs_tree_render is true. Free with command_result_free().
  *
  */
-CommandResult *run_binding(Binding *bind);
+CommandResult *run_binding(Binding *bind, Con *con);