Implement the ipc 'binding' event

The binding event will be triggered when a binding is run as a result of
some a user action. The binding event has the following properties:

change: (str) Currently this will only be "run" but may be expanded in
the future. Included for consistency with other events.

binding: (map) the serialized binding

The "binding" member will have these properties:

input_type: (str) either "keyboard" or "mouse"

input_code: (int) the xcb keycode of the keyboard binding if it was
provided or the mouse button if it is a mouse binding.

symbol: (str) the string representation of the input code

command: (str) the bound command

mods: (list of str) a list of the modifiers that were pressed as string
symbols

fixes #1210

include/ipc.h

@@ -105,3 +105,8 @@ void ipc_send_window_event(const char *property, Con *con);
  * For the barconfig update events, we send the serialized barconfig.
  */
 void ipc_send_barconfig_update_event(Barconfig *barconfig);
+
+/**
+ * For the binding events, we send the serialized binding struct.
+ */
+void ipc_send_binding_event(const char *event_type, Binding *bind);