[Model] Reset requested range on flush()

Without this flush() won't result in a re-fetch as it should.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[UPnP] Add Server-abstraction with a sort_caps-property

This will be useful for deciding which columns are sortable.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[MainWindow] Set columns clickable only if they are sortable

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