Add a way to get the next hotkey pressed

Nobody has claimed this yet.

In certain contexts, when implementing a timer using this lib, the implementor will want to provide the user with a way to press a key which will then be set as a hotkey. However, if the implementor is using an input system which doesn't map surjectively (ie if two keypresses are the same according to the implementor's input system, they are the same according to livesplit-hotkey) to livesplit (eg winit/glutin), this becomes impossible to actually do without registering and then deregistering a hook for every single hotkey.

My proposal to fix this is to add a `register_global_hook` function that will call a callback whenever livesplit-hotkey recieves any keypress and provide that callback with the key that was pressed.

No files or tests are named. Start by locating the existing hotkey registration and hook entry points, then review how keyboard events are exposed across supported platforms. Done means a global hook can report each pressed key and can be registered and removed without requiring one hook per hotkey.

Written by the indexing model from the issue text.