Refactor input handler

[y5nw]

Mostly preparations for allowing more complex keyboard-related features (e.g. secondary keybindings); largely based on #15918 (which turned out to be useful after all after some more changes). The main changes (later ones can be dropped if we want to put them into later PRs that actually need the commits):

• Implement operator< (for e.g. std::set) and std::hash (for std::unordered_map) for KeyPress. KeyList is dropped accordingly.

• The event receiver now stores keypress information into a bitset indexed using GameKeyType. This architecture is useful as it avoids future headaches around e.g. wasKeyDown.

• We continue to keep track of physical keys. This is useful for e.g. secondary keybindings, where (with the above change) we want to make sure that other keybindings are also unset before releasing a GameKeyType key (physical keys are still marked as relased when they are released, of course).

• KeyCache is dropped and integrated into the event receiver, as the changes around the event receiver makes it no longer necessary for the input handler to hold information on keybindings, while it can become useful in the future for the event receiver to quickly look up keypresses (see above point for example).

• Does it resolve any reported issue?
  No.

• Does this relate to a goal in the roadmap?
  The first two commits qualify for 2.2 Internal code refactoring. Other the entire PR also eases development for future feature PRs.

To do

This PR is Ready for Review.

How to test

Check that keyboard input still works.

[sfan5]

(code review only)

[sfan5]

seems to work