[no sq] Various improvements to the input handler

[y5nw]

The order of the commits is more or less outlined in #16750 (comment). I chose to not open 4 pull requests as some commits (particularly the second commit) introduce changes that are obsoleted/removed in later ones.

• First commit: Implement secondary keybindings [UI Proposal 2] #16803 (comment)
• Second commit: Adjustments, among other things:
  • KeyPress::scancode is renamed to KeyPress::value
  • KeyPress::getType() is introduced to get the type of input
  • Calls to std::variant-related functions and methods use indices (based on getType()) instead of types; this makes it easier to infer the type of input.
• Third commit: Adjusted version of Decouple TouchControls from keybinding settings #16215; changes from that PR are split between this commit and the previous commit to ease reviewing
• Fourth commit: Fixes Cannot bind any game controls to mouse buttons 6 and above #16750
  • The EKEY_CODE variant is removed from KeyPress as it is no longer needed
  • I have chosen to not merge LMB/MMB/RMB button events as a lot of (GUI-related) code have different paths based on the specific mouse button while the in-game input handler is practically the only place with a combined code path for all mouse buttons; in other words, there is IMO little to gain from merging LMB/MMB/RMB events.
  • The keycode migration is only handled in Lua. This allows differentiating mouse button migrations from pre-5.12 keycode migrations, the former of which does not require the user to reconfigure keybindings.

To do

This PR is a Work in Progress.

How to test

• First/second commits: 🤷
• Third commit: Unbind keys from the keybinding menu and observe that touchscreen buttons continue to work
• Fourth commit:
  • Bind actions to mouse keys (particularly X1, X2, and beyond) and observe that these continue to work
  • Observe that settings with older mouse buttons (e.g. keymap_autoforward = KEY_MBUTTON) are automatically migrated (i.e. continue to work), and the user is not informed about the migration.