Send IPC modifier event on bar_modifier up/down

Detects when a bar modifier key is pressed/released and sends a modifier
IPC event to any listeners (usually swaybars).

This way a swaybar can listen on the modifier event and hide/show the
bar accordingly (not implemented yet)

The modifier event looks like this:

    {
      "change": "pressed", // or released
      "modifier": "Mod4"
    }

include/ipc.h

@@ -17,6 +17,7 @@ enum ipc_command_type {
 	IPC_EVENT_WINDOW = (1 << 31 | 3),
 	IPC_EVENT_BARCONFIG_UPDATE = (1 << 31 | 4),
 	IPC_EVENT_BINDING = (1 << 31 | 5),
+	IPC_EVENT_MODIFIER = (1 << 31 | 6),
 	IPC_SWAY_GET_PIXELS = 0x81
 };