Move the search interface to a new widget: EShellSearchbar

The current search rule is now stored in EShellView, and the search
context in EShellViewClass similar to GalViewCollection (since it's
class-specific, not instance-specific).

Also add a couple new signals to EShellView: "clear-search" and
"custom-search" ("custom" refers to an advanced search or a saved
search -- something more complex than a quick search).

Still working out a few kinks.  The search entry is clearly trying to
be too many things.  We need a different way of indicating that you're
looking at search results.  Perhaps a search results banner similar to
Nautilus.