Also, Introduce a search_mode_active GObject property in
SearchView. This allows to remove an ambiguity when SearchView is
visible.
On one hand, when accessing a child view from SearchView, the
searchbar becomes invisible (search_mode_active is False) but the
search State is unchanged to prevent the searchbar from being
cleared.
On the other hand, when leaving SearchView, the searchbar
becomes invisible and search State needs to be reset (None) to clear
the searchbar and restore the previous view before the search. This
operation takes place in search_mode_active setter which propagates to
Search and then the main window.

These changes should help simplify search-mode interactions
between the main views and the searchbar.