[UPnP] Add Server-abstraction, use it in MainWindow

Add the whole Server-object into the server-liststore:
This way we don't need to add new columns when we need to use new
properties like SortCaps.

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

Fixes #24 (although whether "unclickable" means "greyed out" depends
on the theme).

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