Refactor input stuff

This is the start of a significant refactoring of everything related to sending messages, which is roughly:

• the chatbox
• action handling
• message sending on the c++ side
• autocompletion of users/rooms/emojis/commands/things i forgot

Notable changes so far include:

• ChatBox is now a ColumnLayout. As part of this, i removed the height animations for now. as far as i can tell, they were broken anyway. I'll readd them later
• Actions were refactored to live outside of the message sending function and are now each an object; it's mostly a wrapper around a function that is executed when the action is invoked
• Everything that used to live in ChatBoxHelper is now in NeoChatRoom; that means that the exact input status (text, message being replied to, message being edited, attachment) is now saved between room switching).
• To edit/reply an event, set NeoChatRoom::chatBox{edit,reply}Id to the desired event id, NeoChatRoom::chatBox{reply,edit}{User,Message} will then be updated automatically
• Attachments behave equivalently with NeoChatRoom::chatBoxAttachmentPath
• Error message reporting from ActionsHandler has been fixed (same fix as in !517 (merged)) and moved to NeoChatRoom

Broken at the moment:

• Any kind of autocompletion
• Mentions
• Fancy effects
• sed-style edits
• last-user-message edits and replies
• Some of the actions, probably
• Replies from notifications
• Lots of keyboard shortcuts
• Custom emojis
• ChatBox height animations

TODO:

• User / room mentions based on QTextCursors instead of the hack we currently use
• Refactor autocompletion stuff
• ???
• Profit