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 <email@example.com>