[GTK] Fixed problem disposing childs navigating back (#3116) Fixes #2987

* Fixed problem disposing childs navigating back

* Fixed text format

Xamarin.Forms.Platform.GTK/Renderers/AbstractPageRenderer.cs

@@ -1,5 +1,6 @@
 using Gtk;
 using System;
+using System.Collections.ObjectModel;
 using System.ComponentModel;
 using Xamarin.Forms.Internals;
 using Xamarin.Forms.Platform.GTK.Extensions;
@@ -90,6 +91,13 @@ namespace Xamarin.Forms.Platform.GTK.Renderers
 			{
 				if (_appeared)
 				{
+					ReadOnlyCollection<Element> children = ((IElementController)Element).LogicalChildren;
+					for (var i = 0; i < children.Count; i++)
+					{
+						var visualChild = children[i] as VisualElement;
+						visualChild?.Cleanup();
+					}
+
 					Page.SendDisappearing();
 				}