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/Menu/Timeline/MessageDelegateContextMenu.qml

@@ -28,13 +28,19 @@ Loader {
         Kirigami.Action {
             text: i18n("Edit")
             icon.name: "document-edit"
-            onTriggered: chatBoxHelper.edit(message, formattedBody, eventId);
+            onTriggered: {
+                currentRoom.chatBoxEditId = eventId;
+                currentRoom.chatBoxReplyId = "";
+            }
             visible: eventType.length > 0 && author.id === Controller.activeConnection.localUserId && (eventType === "emote" || eventType === "message")
         },
         Kirigami.Action {
             text: i18n("Reply")
             icon.name: "mail-replied-symbolic"
-            onTriggered: chatBoxHelper.replyToMessage(eventId, message, author);
+            onTriggered: {
+                currentRoom.chatBoxReplyId = eventId;
+                currentRoom.chatBoxEditId = "";
+            }
         },
         Kirigami.Action {
             visible: author.id === currentRoom.localUser.id || currentRoom.canSendState("redact")