[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;
@@ -82,26 +83,33 @@ namespace Xamarin.Forms.Platform.GTK.Renderers
             return Container.GetDesiredSize(widthConstraint, heightConstraint);
         }
 
-        public override void Dispose()
-        {
-            base.Dispose();
+		public override void Dispose()
+		{
+			base.Dispose();
 
-            if (!_disposed)
-            {
-                if (_appeared)
-                {
-                    Page.SendDisappearing();
-                }
+			if (!_disposed)
+			{
+				if (_appeared)
+				{
+					ReadOnlyCollection<Element> children = ((IElementController)Element).LogicalChildren;
+					for (var i = 0; i < children.Count; i++)
+					{
+						var visualChild = children[i] as VisualElement;
+						visualChild?.Cleanup();
+					}
 
-                _appeared = false;
+					Page.SendDisappearing();
+				}
 
-                Dispose(true);
+				_appeared = false;
 
-                _disposed = true;
-            }
-        }
+				Dispose(true);
 
-        protected override void OnShown()
+				_disposed = true;
+			}
+		}
+
+		protected override void OnShown()
         {
             base.OnShown();
 
@@ -125,7 +133,7 @@ namespace Xamarin.Forms.Platform.GTK.Renderers
 
             _appeared = false;
 
-            PageController.SendDisappearing();
+			PageController.SendDisappearing();
         }
 
         protected override void OnSizeAllocated(Gdk.Rectangle allocation)