Moved all features of UnityApplications QML plugin into the Unity2d QML plugin.

- /usr/lib/qt4/imports/UnityApplications and its content do not exist anylonger.
- All references to UnityApplications have been updated to reflect the change.
  In particular QML imports of UnityApplications have been replaced with imports of Unity2d.
- Build instructions have been transferred from launcher/UnityApplications/CMakeLists.txt
  into libunity-2d-private/Unity2d/CMakeLists.txt.
- Registrations of classes have been transferred from launcher/UnityApplications/plugin.cpp
  into libunity-2d-private/Unity2d/plugin.cpp.
- As a consequence launchermenutest had to be moved into libunity-2d-private/tests