...if the project was updated in the meanwhile.

If a project is updated mark invisible editor documents dirty and update
them if they get focus.

This also fixes document highlighting when restoring a session for
documents that the user "switched away" before the project info is
pushed to CppModelManager.

This completes

    CppTools: Update visible documents on project update
    commit c2eb91e0



which only takes care of visible documents.

Task-number: QTCREATORBUG-13270
Change-Id: Id445e7f509deac5d03194aecc54ce4629b7926ce
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>