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

src/roommanager.cpp

@@ -10,6 +10,7 @@
 #include <QDesktopServices>
 #include <QStandardPaths>
 #include <qt_connection_util.h>
+#include <QQuickTextDocument>
 #include <user.h>
 
 #ifndef Q_OS_ANDROID
@@ -123,6 +124,12 @@ void RoomManager::openRoomForActiveConnection()
 
 void RoomManager::enterRoom(NeoChatRoom *room)
 {
+    if (m_chatDocumentHandler) {
+        // We're doing these things here because it is critical that they are switched at the same time
+        m_currentRoom->setSavedText(m_chatDocumentHandler->document()->textDocument()->toPlainText());
+        m_chatDocumentHandler->setRoom(room);
+        m_chatDocumentHandler->document()->textDocument()->setPlainText(room->savedText());
+    }
     m_lastCurrentRoom = std::exchange(m_currentRoom, room);
     Q_EMIT currentRoomChanged();
 
@@ -232,3 +239,15 @@ void RoomManager::leaveRoom(NeoChatRoom *room)
 
     room->forget();
 }
+
+ChatDocumentHandler *RoomManager::chatDocumentHandler() const
+{
+    return m_chatDocumentHandler;
+}
+
+void RoomManager::setChatDocumentHandler(ChatDocumentHandler *handler)
+{
+    m_chatDocumentHandler = handler;
+    m_chatDocumentHandler->setRoom(m_currentRoom);
+    Q_EMIT chatDocumentHandlerChanged();
+}