Add 'real' compose key

[EvyBongers]

Describe your suggested feature

Right now there's two versions of many keyboards, one 'normal' version and one 'compose' version. The second uses something similar to dead keys in order to insert accented characters, whereas MessagEase had a compose key similar to how that works on Linux. A few example sequences:

• <compose>, s, s -> ß
• <compose>, e, = -> €
• <compose>, u, " -> ü

A list of compose sequences supported by Linux can be found here: https://git.kernel.org/pub/scm/linux/kernel/git/legion/kbd.git/tree/data/keymaps/include

Other details

No response

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