Add idle-inhibitor ipc subscription type

Add a new type of ipc subscription so clients can be notified of changes
in idle inhibitors. This allows to react to those events by e.g.
changing container properties (title, ...) to notify the user about this
circumstance (add marker, change color, ...). For this reason the event
includes information about the affected container as well.

Signed-off-by: Michael Weiser <michael.weiser@gmx.de>

include/ipc.h

@@ -36,6 +36,7 @@ enum ipc_command_type {
 	// sway-specific event types
 	IPC_EVENT_BAR_STATE_UPDATE = ((1<<31) | 20),
 	IPC_EVENT_INPUT = ((1<<31) | 21),
+	IPC_EVENT_IDLE_INHIBITOR = ((1<<31) | 22),
 };
 
 #endif