[PropertyPanel] Adds visibility to customproperty panel initial state

Author: netonjm

main/src/addins/MonoDevelop.DesignerSupport/MonoDevelop.DesignerSupport/PropertyPad.cs

@@ -230,7 +230,10 @@ internal void UseCustomWidget (Gtk.Widget widget)
 			customWidget = true;
 			frame.Remove (frame.Child);
 			frame.Add (widget);
-			widget.Show ();			
+			widget.Show ();
+			if (container != null) {
+				widget.Visible = container.ContentVisible;
+			}
 		}
 
 		void ClearToolbar ()