[backend] New libunity-2d that hosts code formerly in UnityPlaces and libuqpanel.
- Introduce libunity-2d, which contains most of libuqpanel.
- Introduce libunity-2d-qml, which contains a few C++ classes implementing QML objects. These classes come from UnityPlaces.
- Get rid of libuqpanel. Most of it is in libunity-2d, only a few classes went into unity-2d-panel
- Remove dead code from UnityPlaces.