A proper menu system

This update adds a main menu to the game. It shows you controls and credits, and also lets you select a scene. There’s now a (very basic) “basement” scene to go along with the existing house one.

It also probably fixes a few issues, but I didn’t write very detailed notes, whoops…

I’ll be working on an in-game UI next. One little improvement: when you unlock the cursor, your view stops rotating.