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. <del>as far as i can tell, they were broken anyway.</del> 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) and moved to NeoChatRoom


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

TODO:
- [x] User / room mentions based on QTextCursors instead of the hack we currently use
- [x] Refactor autocompletion stuff
- [x] ???
- [x] Profit

imports/NeoChat/Settings/Emoticons.qml

@@ -21,10 +21,7 @@ Kirigami.ScrollablePage {
     ListView {
         anchors.fill: parent
 
-        model: CustomEmojiModel {
-            id: emojiModel
-            connection: Controller.activeConnection
-        }
+        model: CustomEmojiModel
 
         Kirigami.PlaceholderMessage {
             anchors.centerIn: parent
@@ -99,7 +96,7 @@ Kirigami.ScrollablePage {
                         this.fileDialog = openFileDialog.createObject(QQC2.Overlay.overlay)
 
                         this.fileDialog.chosen.connect((url) => {
-                            emojiModel.addEmoji(emojiCreator.name, url)
+                            CustomEmojiModel.addEmoji(emojiCreator.name, url)
                             this.fileDialog = null
                         })
                         this.fileDialog.onRejected.connect(() => {