search: Define SearchResultInterface and implement valid results with it

search: Define SearchResultInterface and implement valid results with it

Since all the search result classes are now GObject classes, we can enforce
the methods we want to have in there (just activate() for now) using an
interface, to make sure they are implementing what we require and to easily
group all the classes that can be used as search results, even though they
are not extending SearchResult.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559