Add API to retrieve generated files.

Add an API to qbs::Project that can be used to retrieve generated
files (as known by the build system up to that point). This will
be used to e.g. get the name of header files generated from ui files
in Qt Creator.

Change-Id: I4ef1b99f511bb67f14029c90102630c0d498f1e5
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>