Add an emoji layout

Based on GTKs emojichooser. Needs more testing and we could also

• see if we can make the emoji data smaller, as we don't do search we don't need all of it (if we don't want to add completion for it)
• hide recent box when it's empty (or rather make it insensitive)
• recent emoji sort order
• css styling
• disable swipe between keyboard layouts as this as this toggles too easily

but let's put it out for inital testing. (Another month old MR decrustified)

Demo: https://social.librem.one/@agx/109445029220794083