search-entry: Handle find-as-you-type activation internally

search-entry: Handle find-as-you-type activation internally

To enable find-as-you-type when entering the overview and disabling
it when leaving, we used a chain of functions calls from ViewSelector
over SearchTab to SearchEntry. As find-as-you-type should be enabled
while the overview is shown, the activation/deactivation can be
handled entirely by the SearchEntry itself by tying it to the entry's
visibility.

https://bugzilla.gnome.org/show_bug.cgi?id=642196