Cleanup QML imports and license headers

- Group the imports into Qt, KDE, NeoChat
- Import the latest versions
- Remove unused imports
- Remove unused components
- Unify license header styling

imports/NeoChat/Dialog/OpenFileDialog.qml

@@ -1,9 +1,7 @@
-/**
- * SPDX-FileCopyrightText: 2019 Black Hat <bhat@encom.eu.org>
- *
- * SPDX-License-Identifier: GPL-3.0-only
- */
-import QtQuick 2.12
+// SPDX-FileCopyrightText: 2019 Black Hat <bhat@encom.eu.org>
+// SPDX-License-Identifier: GPL-3.0-only
+
+import QtQuick 2.15
 import Qt.labs.platform 1.1
 
 FileDialog {