QbsProjectManager: Don't offer "Add File" functionality for folders.

It does not make sense to add files to anything besides products and
groups.

Task-number: QTCREATORBUG-14286
Change-Id: Iced8cefc4eff3857e9a1a6d3a3a9311dcbb44f11
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>