Implement secondary keybindings

[y5nw]

This PR allows players to define secondary keybindings, e.g. using left or right shift for sneaking in the SDL build.

This PR depends on #15152.
Note that this PR had some minor conflicts with #14964. IMO we should merge that PR first and then this one.

• Goal of the PR
  Allow defining secondary keybindings (see description above).
• How does the PR work?
  • Key settings are represented std::vector<KeyPress>. This allows keeping an order for keypresses (e.g. when saving the settings); it should not be problematic in terms of performance as the list of keybindings for a single setting will likely be very short (containing at most 2 entries) in most real-world cases.
  • Each entry in the string representation is delimited by the | character. It is not possible to escape |. This keep things simple at the cost of not allowing the | key in a key list if the key cannot be represented with a EKEY_CODE, but it should not be problematic as SDL: Use scancodes for keybindings #14964 implements a new keycode syntax that is used by default (and does not involve the | character). Note that a special case is created if the setting string includes only the | character, in which case it is parsed as a key instead of a list of keys.
• Does it resolve any reported issue?
  No.
• Does this relate to a goal in the roadmap?
  No.
• If not a bug fix, why is this PR needed? What usecases does it solve?
  SDL: Use scancodes for keybindings #14964 (comment)

To do

This PR is a Work in Progress.

• Implement handling secondary keybindings
• Update keybinding GUI. (Waiting for Rework guiKeyChangeMenu #15152)

How to test

Change a key setting to have the form of (e.g.) KEY_KEY_Z|KEY_KEY_Y and observe that pressing (in this case) the Z or Y key performs the relevant action. Note that it is currently not possible to specify secondary keybindings in the "Controls" form/GUI (see To-Do).