Refactor the state machine in binaries/data/mods/public/gui/session/input.js

[Rolf Sievers]

In the input.js file, there is a comment which reads:

// TODO: it'd probably be nice to have a better state-machine system, with guaranteed
// entry/exit functions, since this is a bit broken now

I'd like to fix that, this ticket will document the progress and the discussion.

This ticket is blocked by #2407. (Because I won't drastically modify the file structure before you commit my patch over there.)