[AppWell] Fix D&D for ShellApp

The drag and drop case needed to be updated to use ShellApp
correctly.  Export _is_transient for better compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=598227

src/shell-app.h

@@ -34,6 +34,7 @@ const char *shell_app_get_id (ShellApp *app);
 ClutterActor *shell_app_create_icon_texture (ShellApp *app, float size);
 char *shell_app_get_name (ShellApp *app);
 char *shell_app_get_description (ShellApp *app);
+gboolean shell_app_is_transient (ShellApp *app);
 
 ShellAppInfo *shell_app_get_info (ShellApp *app);