Use po for overlay layout translations too

This MR aims to improve translation handling by avoiding to maintain this by a custom method in squeekboard but rather rely on po and GnomeXkbInfo where possible so translators can also use their well known tooling. Every bit here helps (e.g. with the first commit we can already have the overlays translated via po). The rest is basically cleanup from there but also makes the decision to drop custom translations completely which I consider a good thing but I might missing cases where this is still necessary. So if that should stay I can easily split up the MR). Likely best reviewed via git log --reverse -p -w master..HEAD. Some details to each commit:

• popover: Move Emoji and Terminal to ui file: moves the overlay keyboards (emoji and terminal) into the ui file so they can be picked up by po without further effort. Aims to simplify the menu item selection while at that. Hopefully didn't break something.
• popover: Add translator notes leverage a po feature to add some meaning to the single word bits
• popover: Make the ui file match the code file name consistency only
• Remove emoji and terminal from translations The come in via po now and hence can be dropped
• popover: Don't complain about missing translations We get most of it via po now
• Drop custom translation handling With the overlay translations out of the way and GnomeXkbInfo doing a good job, we can rely on it or fix there. No need to hand maintain these (which let to some translations not being loaded at all)
• Drop locale_config Now unused
• Remove custom translations Code doesn't use them so we can drop them.

I like the diffstat

20 files changed, 67 insertions(+), 886 deletions(-)

Closes: #316 (closed)