Handle the generation of the list of files in a project globally, based
on the project tree.

Creator now has the concept of TreeManagers which can enrich the project
tree with additional data (e.g. the files found in a resource file), which
the project does not necessarily know about. So use that tree to find
the files that belong to a project instead of implementing similar features
in each project.

Change-Id: Ia375a914a1f2c0adaa427f9eda834eec2db07f68
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Marco Benelli <marco.benelli@qt.io>