Add focus awareness support

[m4gr3d]

Fixes issue #92

Part 1 of the implementation.
The second part needs to happen in the Godot engine, and involve adding an export option for focus awareness support.

[BastiaanOlij]

Haven't tried running the code but it looks good to me, so we're just adding signals here to when the HMD has become available and when we get focus or not, I guess that last one for when we switch to the home/settings interface and back?

[m4gr3d]

The focus awareness one is tricky to test since according to the documentation, it requires that the app be approved by Oculus for access to the api on the headset.
If the app is approved, then the home menu does appear in the app and the events are dispatched synchronously. Otherwise, the app exits to the home menu as usual, and the events are only dispatched when the user gets back to it.

Nonetheless, having the implementation makes us future-proof in case they decide to expand its access.