eo: make the parent a optional constructor

this is needed in order to sanely bind parent calls to some objejcts,
but not all.

However, from the canvas structure up we *need* a parent, so we can
fetch the evas from it. So declare it there a none optional

Reviewed-by: Felipe Magno de Almeida <felipe@expertisesolutions.com.br>
Differential Revision: https://phab.enlightenment.org/D11734

src/lib/eo/efl_object.eo

@@ -432,6 +432,9 @@ abstract Efl.Object
         class.constructor;
         class.destructor;
    }
+   constructors {
+        .parent @optional;
+   }
    events {
       del @hot: void; [[Object is being deleted. See @.destructor.]]
       invalidate @hot: void; [[Object is being invalidated and losing its parent. See @.invalidate.]]

src/lib/evas/canvas/efl_canvas_object.eo

@@ -483,6 +483,9 @@ abstract Efl.Canvas.Object extends Efl.Loop_Consumer implements Efl.Gfx.Entity,
          }
       }
    }
+   constructors {
+        Efl.Object.parent;
+   }
    implements {
       Efl.Object.constructor;
       Efl.Object.finalize;