Convert physical_keycodes to keycodes in the current keyboard layout

[Frixuu]

As far as I know, there is currently no way to map InputEventKey.physical_keycodes to regular scancodes, using the user's current keyboard layout. This functionality could be used in both the editor and games, providing context-aware button prompts.

This PR aims to explore this topic.

Assumptions

A functional system would take a Key, a Godot enum, in the QWERTY layout. If it is reasonable to provide a Key in the user's current keyboard layout (QWERTZ, Dvorak), it should do so. If it is not (Arabic, Cyrillic scripts), the function should return back the original key. The method should ideally return the same keycode, as it would appear in the handled key-press event.
Note: providing a Unicode symbol for the key is not a goal here.

Platform status

• Linux
  • On layouts with Latin-1, but not ASCII characters, resulting keys might have no corresponding constant. For example on AZERTY, ' (0x27) will map to ù (0xF9). However, this seems consistent with the keycodes in processed input events, and changing that behaviour is not in the scope of this PR.
• macOS
  • Uses the same routines as the input handler. While not all keys seem to be mapped, it is consistent with the incoming events.
  • Do Macs know what a Pause key is?
• Windows
  • To the best of my knowledge, the Windows implementation is the only one that slightly differs from the handled input events. The change is best seen in the case of the AZERTY layout.
    • A physical slash (0x2F) carries a KEY_UNKNOWN, however it gets converted to an exclamation mark (0x21).
    • It maps a period (0x2E) to a colon (0x3A), but the incoming event has a keycode of slash (0x2F).
    • The event contains a backslash (0x5C), but it gets mapped to an asterisk (0x2A).
  • However, given the intended purpose of the proposed function, I would argue that it is not a problem worth solving right now. (it would incur changing other parts of the DisplayServer)

Other

• Come up with some better names
  • For the purpose of this PR, I have picked keyboard_get_keycode_from_physical. I feel it is a bit too verbose while not conveying the intent of the method very clearly. How could we fix it? What verb would be more appropriate?

[akien-mga]

Looks good to me. Thanks for the detailed overview of assumptions and implementation details!

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉