swaybar: show hidden bar on key event

Since wayland does not currently allow swaybar to create global
keybinds, this is handled within sway and sent to the bar using a custom
event, so as not to pollute existing events, called bar_state_update.

include/ipc.h

@@ -30,6 +30,9 @@ enum ipc_command_type {
 	IPC_EVENT_BINDING = ((1<<31) | 5),
 	IPC_EVENT_SHUTDOWN = ((1<<31) | 6),
 	IPC_EVENT_TICK = ((1<<31) | 7),
+
+	// sway-specific event types
+	IPC_EVENT_BAR_STATE_UPDATE = ((1<<31) | 20),
 };
 
 #endif