Avoid more GFile<>uri roundtrips

Make GtkSearchHit carry a GFile instead of an uri. Most of the
search engines already have the object around, and converting
to an uri and back is unnecessary extra work.

gtk/gtksearchengine.h

@@ -42,7 +42,7 @@ typedef struct _GtkSearchHit GtkSearchHit;
 
 struct _GtkSearchHit
 {
-  gchar *uri;
+  GFile *file;
   GFileInfo *info; /* may be NULL */
 };