Add initial po support

This adds enough po/gettext support so that we can get "Keyboard Settings" translated. It uses a bit of an unconventional approach of reusing an unused (AFAIKT) ui file to keep the translations that are otherwise burried in an escaped string in a rust file.

Getting rid of data/langs isn't hard from here (and it will then also be possible to just reuse the pop.ui file and more importantly allow us to get rid of a hardcoded list of languages for translations) but that's more than I want to do today - but I'll follow up once the list of MRs clears.

Once that is merged, I'll ask the i18n team to add this to the list of translatable projects.

Closes: #315 (closed)