search-results: Fix flickering of the selection

When updating search results, the current result set is
recreated from scratch before setting the selection
highlight. This results in two style changes of the selected
item, and as a CSS transition is used to animate the style
change, the selected item flickers if it remains the same as
with the previous search term.
Fix by hiding the result set until the selection is set, to
avoid the transition in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=646019