vk: Build a keymap for character popovers

This allows us to use all symbols in character popovers when in virtual keyboard mode without having to deal with the per language keymaps.

@kailueke this should make the other characters work in popovers of the e.g. the German and Terminal layouts.

Funny enough this e.g. makes the popovers for Ukrainian work but not yet for the "main" layouts. We likely want to use a similar approach for the main layouts as well when we don't have a fine tuned one.