efl_ui_win: this object calls composite_attach for efl.config

Summary:
ref T7652

Depends on D7732

Reviewers: cedric, zmike, stefan_schmidt

Reviewed By: zmike

Subscribers: #reviewers, #committers

Tags: #efl

Maniphest Tasks: T7652

Differential Revision: https://phab.enlightenment.org/D7733

src/lib/elementary/efl_ui_win.eo

@@ -871,4 +871,7 @@ class Efl.Ui.Win extends Efl.Ui.Widget implements Efl.Canvas.Scene, Efl.Access.W
       pause: void; [[Called when the window is not going be displayed for some time]]
       resume: void; [[Called before a window is rendered after a pause event]]
    }
+   composite {
+      Efl.Config;
+   }
 }