i3/ipc: implement reception of arbitrary IPC events in QML

[3akev]

This PR exposes the subscribe call to the QML API, allowing clients to receive subscribed events directly through rawEvent.

My use case is to receive input events in order to detect keyboard layout changes. See here and here for an example doing this.

[outfoxxed]

The intention here is fine but it probably shouldn't be implemented like this, because subscriptions are not dropped between reloads. I would prefer to see something more declarative, e.g. I3IpcListener { subscriptions: []; onMessage: ... } unless you have a particular reason to do it this way.

[3akev]

Implemented a new QML type, as suggested. Here's a usage example. Let me know if further changes are needed.