search: add a panel to configure search providers

The panel lists all the search providers registered on the system, and
allows to reoder or disable them, or disable the search providers
feature entirely.

The panel will also allow configuration, for which a first
implementation will be added in a separate commit.

https://live.gnome.org/Design/SystemSettings/Search

https://bugzilla.gnome.org/show_bug.cgi?id=687490