Initial completion support

In order to allow for text completion this adds a PosCompleter interface that fills in completions that are selectable from a PocCompletion bar. I've "implemented" an example completer based on fzf but completers via e.g. hunspell would work just as well (or even via ibus). We only implement on normal input purpose.

This is more a demo at this point in time (but works for me):

• we need to handle surrounding-text so we pick up the right word to complete when deleting backward into the previous word
• we need to double buffer input-method state correctly
• the completer has lots of stuff hardcoded (it's really just a demo)

Demo: https://social.librem.one/@agx/109286561751410903