Refactor qbs project files.

- Use sub-projects.
- Move the main application and the application header products
  into dedicated files.
As a result, qtcreator.qbs becomes much simpler to understand,
consisting now of just a bunch of global properties and a few sub-project
references.

Change-Id: Ic7300d489d3982394be5ad1d12a79a2914320298
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>