Feature request: Compose key

[Julow]

Related issues: https://github.com/Julow/Unexpected-Keyboard/issues/30 https://github.com/Julow/Unexpected-Keyboard/issues/171 https://github.com/Julow/Unexpected-Keyboard/issues/191 https://github.com/Julow/Unexpected-Keyboard/issues/164

The compose key is a good way to type rarely needed characters. For example, Compose, -, > types →, Compose, ', e types é.

Key combinations are generally intuitive but the keyboard has the opportunity to show the possible combinations during typing, making them easy to discover.

This feature is called Multi_key in XKB and the list of combinations is huge