Move the qt models to their own folder

Felt like the src folder was getting a bit crowded so move all the models to a folder named models.

src/models/messagefiltermodel.h

@@ -0,0 +1,14 @@
+// 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:
+    MessageFilterModel(QObject *parent = nullptr);
+    bool filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const override;
+};