A Compose key instead of Copilot

This gist contains the necessary files to transform the Copilot key into a Compose key under X or
Wayland. This is done by creating a custom keyboard option that simply needs to be enabled in your
system's settings.

Bonus: if your keyboard has a level 3 key (AltGr) then AltGr+Copilot gives you greek letters!

How to:

1. Place each files in `~/.config/xkb/<folder indicated on the first line>`.

2. Restart your X/Wayland session (simply logout then login)

3. Enable the option using your system's setting software. (In KDE's system settings app it is
   located under Keyboard > Key bindings (top right corner) > Position of Compose key, check the box
   "Compose on Copilot, greek on AltGr+Copilot")
   
4. Enjoy having made the Copilot key useful. Tip: `/usr/share/X11/locale/<your locale>/Compose`
   lists the available compose sequences. (<your locale> defaults to `en_US.UTF-8` if the
   corresponding folder does not exists)

NOTES:

- Despite the name of everything starting with X, this indeed works for Wayland.
  (Short story: Wayland doesn't have its own way to handle keyboard layout so it reuses things from X)
  
- I assume that you use a recent version of X/Wayland that uses libxkbcommon to handle keyboard
  layouts. If yours doesn't, you may need to edit the files under `/usr/share/X11/xkb` as root.
  However, beware that every update of the package `xkeyboard-config` may override your changes.

- This does not actually remap the copilot key but rather the combination super+shift+F23 because
  this is how Copilot keys are physically wired inside keyboards.
  
- I made this almost exclusively using the documentation for libxkbcommon (https://xkbcommon.org/doc/current/),
  in particular the section "User-configuration". So if you want more info on how this works, head
  over there.

compose

// Place this in the `symbols` folder

partial alphanumeric_keys modifier_keys
xkb_symbols "copilot" {
    key <FK23> {
        type[Group1] = "CUSTOM_SHIFT_SUPER_ALTGR",
        [ NoSymbol, Multi_key, dead_greek ]
    };
};

custom

// Place this in the `types` folder

default partial xkb_types "default" {

    virtual_modifiers LevelThree, Super;

    type "CUSTOM_SHIFT_SUPER_ALTGR" {
        modifiers = Shift + LevelThree + Super;
        map[None] = Level1;
        map[Super + Shift] = Level2;
        map[Super + Shift + LevelThree] = Level3;
        level_name[Level1] = "Base";
        level_name[Level2] = "Super";
        level_name[Level3] = "Super AltGr";
    };
};

evdev

// Place this in the `rules` folder

! include %S/evdev

! option          = symbols
  compose:copilot = +compose(copilot)

! option          = types
  compose:copilot = +custom

evdev.xml

<!-- Place this in the `rules` folder -->

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xkbConfigRegistry SYSTEM "xkb.dtd">
<xkbConfigRegistry version="1.1">
  <optionList>
    <group allowMultipleSelection="true">
      <!-- Extend the options for the compose key -->
      <configItem>
        <name>Compose key</name>
      </configItem>
      <option>
        <configItem>
          <name>compose:copilot</name>
          <description>Compose on Copilot, greek on AltGr+Copilot</description>
        </configItem>
      </option>
    </group>
  </optionList>
</xkbConfigRegistry>