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).
• ...?

Bug list:

• The 'no confirm' hotkey doesn't properly work with all combinations, only the first, which means on mac that to kill without confirm one needs to use 'Fn', which isn't needed otherwise.