Add configurable keybindings

[kpreid]

Add a way to customize the key (and mouse button) bindings. Look into if there is an existing library for the job. If not, maybe write one that is separate from Cubes.

UI design sketch: List all the functions, have a widget you click on followed by the desired key/button (focus it, have it catch all events). For an already bound function, have an "+" button which lets you specify additional bindings.

Extras:

• While we're at it, add auto-repeat for the jump and place/delete block functions.
• Future: look into gamepad support (there is an experimental browser API for that.

[kpreid]

Done in commit 6778ef1bb5a7a20bc57035c4d1997900b961942e, without the extras.

[kpreid]

Extras filed as #50 and #51.