QbsProjectManager: Provide functionality to add files to a Qbs project.

Started implementing common functions to add new and existing files to
a project, using the API provided by Qbs. Works for most use cases
already. However, this is still work in progress.

Change-Id: I36ea921c203bd7caad564a45cd40cda408c72b54
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Qbs: Factor parsing code out of QbsProject

Change-Id: I9b86baf964252e3d155cce533c79ca7a9dc9b315
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>