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/completionproxymodel.h

@@ -0,0 +1,33 @@
+// SPDX-FileCopyrightText: 2022 Tobias Fella <fella@posteo.de>
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#pragma once
+
+#include <QSortFilterProxyModel>
+
+class CompletionProxyModel : public QSortFilterProxyModel
+{
+    Q_OBJECT
+    Q_PROPERTY(int secondaryFilterRole READ secondaryFilterRole WRITE setSecondaryFilterRole NOTIFY secondaryFilterRoleChanged)
+    Q_PROPERTY(QString filterText READ filterText WRITE setFilterText NOTIFY filterTextChanged)
+
+public:
+    bool filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const override;
+
+    int secondaryFilterRole() const;
+    void setSecondaryFilterRole(int role);
+
+    QString filterText() const;
+    void setFilterText(const QString &filterText);
+
+    void setFullText(const QString &fullText);
+
+Q_SIGNALS:
+    void secondaryFilterRoleChanged();
+    void filterTextChanged();
+
+private:
+    int m_secondaryFilterRole = -1;
+    QString m_filterText;
+    QString m_fullText;
+};