Consider FCITX support

[Mindful]

Would ideally like to support Cosmic desktop when it comes out, but not clear if Cosmic will support IBUS.

FCITX seems reasonably well documented (at least better than IBUS, maybe), although I need to look into it more. https://fcitx-im.org/wiki/Develop_an_simple_input_method

Edit: although obviously this would be a big undertaking relative to the size of the project, since half the code right now is just integrating with IBUS.