shell: Keep the search button on the right panel

The search isn't global, it's only for the Explore and Installed page,
so it doesn't make much sense for it to be on the top-level panel. Even
more since the search bar itself is also on the right panel, so moving
it makes the right panel more consistent.
3 jobs for search-right-panel in 8 minutes (queued for 3 minutes and 14 seconds)