Allow bulk changes to the settings model data.

Intended use case is to allow users of Qt Creator to supply a
set of properties that add to or override what's currently in
the settings.

Change-Id: I4355763905eca97f63e90f9c8320d77e6a99dfe5
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>