Use a qbs-specific group for the settings.

Otherwise, non-qbs settings from fallback locations will add noise for
users. This patch is in effect an addendum to commit 065d89fa2c, which
assumed that this happened only on OSX. However, we have now seen such
problems also on Linux.

Change-Id: If4e40d5941c8e1bd249cf75096b59d4b10880d00
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>