ipc: add an input event

This adds an ipc event related to input devices. Currently the
following changes are supported:
- added: when an input device becomes available
- removed: when an input device is no longer available
- xkb_keymap_changed: (keyboards only) the keymap changed
- xkb_layout_changed: (keyboards only) the effective layout changed

include/ipc.h

@@ -34,6 +34,7 @@ enum ipc_command_type {
 
 	// sway-specific event types
 	IPC_EVENT_BAR_STATE_UPDATE = ((1<<31) | 20),
+	IPC_EVENT_INPUT = ((1<<31) | 21),
 };
 
 #endif