GtkPlacesSidebar: Fix Home and Desktop on Windows.

We were concatenating "file://" to the result of g_get_user_special_dir() to build
a URI, but this is not enough on Windows.  Use g_filename_to_uri() instead.

See https://bugzilla.gnome.org/show_bug.cgi?id=739453

gtk/gtkplacessidebar.c

@@ -690,7 +690,7 @@ get_home_directory_uri (void)
   if (!home)
     return NULL;
 
-  return g_strconcat ("file://", home, NULL);
+  return g_filename_to_uri (home, NULL, NULL);
 }
 
 static gchar *
@@ -706,7 +706,7 @@ get_desktop_directory_uri (void)
   if (path_is_home_dir (name))
     return NULL;
 
-  return g_strconcat ("file://", name, NULL);
+  return g_filename_to_uri (name, NULL, NULL);
 }
 
 static gboolean