Added basic inventory and UI

- Added inventory, clicking a pickable item will move it to the
inventory and UI layer
- Added a camera just for UI, rendering only things in the UI layer.
- Clicking an inventory item will remove it (just for testing),
remaining items slide into place
- PlayerMouseControl uses a new ray on the UI camera to get clicks in UI
stuff
- Removed the little hover text from item prefab, added a single,
centered 3D text that changes when you hover over an interactable.

NB! Didn't think too far ahead for much of this, feel free to change it
around/scraps parts we don't need  :)