Improve mouse handling, new terminal sequences needed?

[cryptocode]

Terminal Doom currently plays best in keyboard-only mode, though mouse is workable if you get sensitivity just right + a lot of practice.

However, playing with the mouse will likely never be optimal until terminals gain a better standard for mouse handling.

In particular, we need:

1. A way to capture the mouse and get relative mouse move events even when running off the edge
2. A way to hide the mouse cursor while the game is playing