Add emoji picker for inserting unicode and custom emojis

Based off of the emoji picker from NeoChat, but slightly specialized to Mastodon's requirements. Adds a new EmojiModel, and we now load custom emoji from the server. For now, the emojis do not animate.

• The QML is mostly unchanged, but I ripped out all of the stuff we don't need (sticker support, custom emojis are always supported, etc.)
• The emoji model is much, much more lean than NeoChat's. I wonder if NeoChat is even using half of that class 🤔
• The emoji model class in NeoChat used QSettings to store last used emojis, our version stores it in the new tokodon state config! This is per-account of course, since different accounts could be on different servers with different emojis, etc.
• Slight improvements to the picker performance, my server has quite a bit of emojis and now it's nice and smooth. Not sure why animated emojis do not playback yet, but since we don't really support them anyway (#52 (closed)) I think that's alright for now.
• The custom emojis are not categorized yet, but at least you can search through then :-)