[Model] Add "static rows" in the model

This allows inserting a static row into the model. It will be
inserted before the actual search/browse results. This is useful
for BrowseModel that wants to edd e.g. ".."-row for moving up a
directory

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[Browse] Use the GenericModel for Browse

This removes a lot of duplicate code and makes Browse perform as well
as Search.

Note that BrowseModels are discarded as soon as the view changes to
another "directory" (BrowseModel). Caching could be added later on.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>