Improved (and simplified) the stub directory structure

- Removed the "cpp/" dir
- Moved main.cpp and symbianicon.svg next to the main .pro.
  This also reduces the dir depth in the project view.
- Introduced "qmlapplicationviewer/" dir, which will contain the files
  which the user should not modify. All the files there have the base
  name "qmlapplicationviewer"
- Moved/renamed "qmlapplication.pri" to
  "qmlapplicationviewer/qmlapplicationviewer.pri"