Rewrite btscanner example in QML

https://codereview.qt-project.org/c/qt/qtconnectivity/+/467417 moves the btscanner example to manual tests. This is done because the example is QWidget-based, and so it's not fully functional on mobile platforms.

Specifically, there is no way to open a context menu, and to trigger the QListWidget::itemActivated() signal.

However, the example might still be useful for doing Classic Bluetooth scan. It is also the only example that demonstrates device pairing.

The idea of the task is to re-introduce the example, providing a QML UI for it. The non-UI part of C++ code should stay the same. Probably the only reasonable update is to limit the scan to only Classic devices, because we have a separate example for LE scan.