Draft: Use uim for Japanese input

Based on !8 (merged) use the completion infrastructure to wire up uim. Things that certainly need to be done:

• Make the jp.json layout actually look like one
• make the completion bar not overflow when selecting kanji
• reset input method on focus in/out
• don't hardcode completer but select by keyboard layout

I'd appreciate any input (and certainly code too) to make this useful.

If this looks like the wrong approach then please let me know too, I don't speak Japanese myself.

Demo: https://social.librem.one/@agx/109326021241539147