Composer: doesn't play nicely with OS-level input enhancements

[lampholder]

Inspired by #7440, aggregating the various composer/keyboard issues into a meta issue (these issue I think are not related to the "dead keys don't work" bug):

• [OSX][QWERTY] Emphasized character preceded by the unaccented character #7419
• Inserting emoji using OS X character picker causes composer to lose focus #7218
• native macos keyboard text replacement (substitution) are not working on riot client #7155
• Character error #6101 (maybe)

[aaronraimist]

#7527

[lampholder]

#7535

[turt2live]

#7488

[turt2live]

#7639

[turt2live]

I think these are all ianstormtaylor/slate#2368