Move message filtering to C++

The filter callback is called very often (O(messages)). The current
filter model shows some significant overhead in QML internals. Moving
that to C++ makes it quite a bit faster.

src/messagefiltermodel.h

@@ -0,0 +1,15 @@
+/*
+ * SPDX-FileCopyrightText: 2021 Nicolas Fella <nicolas.fella@gmx.de>
+ * SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL
+ */
+#pragma once
+
+#include <QSortFilterProxyModel>
+
+class MessageFilterModel : public QSortFilterProxyModel
+{
+    Q_OBJECT
+public:
+    bool filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const override;
+
+};