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/i3/ipc.h

@@ -100,3 +100,6 @@ typedef struct i3_ipc_header {
 
 /** Bar config update will be triggered to update the bar config */
 #define I3_IPC_EVENT_BARCONFIG_UPDATE (I3_IPC_EVENT_MASK | 4)
+
+/** The binding event will be triggered when bindings run */
+#define I3_IPC_EVENT_BINDING (I3_IPC_EVENT_MASK | 5)