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.cpp

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: 2022 Tobias Fella <fella@posteo.de>
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#include "completionproxymodel.h"
+#include <QDebug>
+
+#include "neochatroom.h"
+
+bool CompletionProxyModel::filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const
+{
+    Q_UNUSED(sourceParent);
+    if (m_filterText.isEmpty()) {
+        return false;
+    }
+    return (sourceModel()->data(sourceModel()->index(sourceRow, 0), filterRole()).toString().startsWith(m_filterText)
+            && !m_fullText.startsWith(sourceModel()->data(sourceModel()->index(sourceRow, 0), filterRole()).toString()))
+        || (m_secondaryFilterRole != -1
+            && sourceModel()->data(sourceModel()->index(sourceRow, 0), secondaryFilterRole()).toString().startsWith(m_filterText.midRef(1)));
+}
+
+int CompletionProxyModel::secondaryFilterRole() const
+{
+    return m_secondaryFilterRole;
+}
+
+void CompletionProxyModel::setSecondaryFilterRole(int role)
+{
+    m_secondaryFilterRole = role;
+    Q_EMIT secondaryFilterRoleChanged();
+}
+
+QString CompletionProxyModel::filterText() const
+{
+    return m_filterText;
+}
+
+void CompletionProxyModel::setFilterText(const QString &filterText)
+{
+    m_filterText = filterText;
+    Q_EMIT filterTextChanged();
+}
+
+void CompletionProxyModel::setFullText(const QString &fullText)
+{
+    m_fullText = fullText;
+}