Implement secondary keybindings

[y5nw]

Reworked version of #15577, written in a way that reduces conflict with future work (e.g. for keybindings with modifiers). The first few commits are separated into #15933 for easier review.

This PR depends on (and is also rebsaed on top of) #15791.

• Goal of the PR
  Allows players to define secondary keybindings, e.g. left and right shift for the same action.
• How does the PR work?
  (See Refactor input handler #15933 for the first few commits.)
  It introduces the syntax of key1|key2, with key1 and key2 being keysyms.
  On top of the first few commits, the event handler is changed to recheck whether an action should be "pressed" or not. (Note that the logic for pressing the key is not short-circuited to set the action - this is helpful for a later PR to implement modifier 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?
  E.g. SDL: Use scancodes for keybindings #14964 (comment)

To do

This PR is a Work in Progress.

• Implement secondary keybindings.
• Update GUI (waiting on Move keybinding settings to (Lua-based) setting menu #15791).
• Idea for a followup PR: Decouple touchscreen buttons from keyboard input?

How to test

Add e.g. keymap_right = SYSTEM_SCANCODE_7|SYSTEM_SCANCODE_9 and observe that both the D and F keys move the player to the right.