TabbedRenderer / load icons from custom sources (from embbedded svg for example) (#574)

* Enable ios tab bar icons to be set by a custom renderer (using xamsvg for example)

* Fix space => tabs

* space => tabs / newline

* space / newlines

Xamarin.Forms.Platform.iOS/Renderers/TabbedRenderer.cs

@@ -370,7 +370,8 @@ namespace Xamarin.Forms.Platform.iOS
 		void UpdateCurrentPage()
 		{
 			var count = ((IPageController)Tabbed).InternalChildren.Count;
-			((TabbedPage)Element).CurrentPage = SelectedIndex >= 0 && SelectedIndex < count ? Tabbed.GetPageByIndex((int)SelectedIndex) : null;
+			var index = (int)SelectedIndex;
+			((TabbedPage)Element).CurrentPage = index >= 0 && index < count ? Tabbed.GetPageByIndex(index) : null;
 		}
 
 		void IEffectControlProvider.RegisterEffect(Effect effect)
@@ -384,17 +385,32 @@ namespace Xamarin.Forms.Platform.iOS
 			if(page == null)
 				throw new InvalidCastException($"{nameof(renderer)} must be a {nameof(Page)} renderer.");
 
-			UIImage icon = null;
-			if (!string.IsNullOrEmpty(page.Icon))
-				icon = new UIImage(page.Icon);
-
-			renderer.ViewController.TabBarItem = new UITabBarItem(page.Title, icon, 0)
+			var icons = GetIcon(page);
+			renderer.ViewController.TabBarItem = new UITabBarItem(page.Title, icons?.Item1, icons?.Item2)
 			{
 				Tag = Tabbed.Children.IndexOf(page),
 				AccessibilityIdentifier = page.AutomationId
 			};
-
-			icon?.Dispose();
+			icons?.Item1?.Dispose();
+			icons?.Item2?.Dispose();
+		}
+		
+		/// <summary>
+		/// Get the icon for the tab bar item of this page
+		/// </summary>
+		/// <returns>
+		/// A tuple containing as item1: the unselected version of the icon, item2: the selected version of the icon (item2 can be null),
+		/// or null if no icon should be set.
+		/// </returns>
+		protected virtual Tuple<UIImage, UIImage> GetIcon(Page page)
+		{
+		    if (!string.IsNullOrEmpty(page.Icon))
+		    {
+		        var icon = new UIImage(page.Icon);
+		        return Tuple.Create(icon, (UIImage)null);
+		    }
+		
+		    return null;
 		}
 	}
 }