Further hotkey features to implement

[wraitii]

Meta-ticket following r24215.

There are a number of improvements to the hotkeys still possible. i'm milestoning this for A24 but it's a likely pushback.

Feature list (from the top of my head):

• More platform support (e.g. Delete on mac should translate to Backspace, really).
• Give hotkeys a better name and a description so that players aren't confused (cf in-game Manual).
• Better way to categorise hotkeys (right now, it's the name prefix which is not ideal).
• ...?