Merge branch '3.6.0' into 4.0.0

diff --cc Xamarin.Forms.Core/Properties/AssemblyInfo.cs
index 2e63daa,a10fa3d..e77182f
--- 1/Xamarin.Forms.Core/Properties/AssemblyInfo.cs
--- 2/Xamarin.Forms.Core/Properties/AssemblyInfo.cs
+++ b/Xamarin.Forms.Core/Properties/AssemblyInfo.cs
@@@ -40,10 -40,12 +40,12 @@@ using Xamarin.Forms.StyleSheets
  [assembly: XmlnsPrefix("http://xamarin.com/schemas/2014/forms/design", "d")]
  
  [assembly: StyleProperty("background-color", typeof(VisualElement), nameof(VisualElement.BackgroundColorProperty))]
 -[assembly: StyleProperty("background-image", typeof(Page), nameof(Page.BackgroundImageProperty))]
 +[assembly: StyleProperty("background-image", typeof(Page), nameof(Page.BackgroundImageSourceProperty))]
  [assembly: StyleProperty("border-color", typeof(IBorderElement), nameof(BorderElement.BorderColorProperty))]
  [assembly: StyleProperty("border-radius", typeof(ICornerElement), nameof(CornerElement.CornerRadiusProperty))]
- [assembly: StyleProperty("border-radius", typeof(IBorderElement), nameof(BorderElement.CornerRadiusProperty))]
+ [assembly: StyleProperty("border-radius", typeof(Button), nameof(Button.CornerRadiusProperty))]
+ [assembly: StyleProperty("border-radius", typeof(Frame), nameof(Frame.CornerRadiusProperty))]
+ [assembly: StyleProperty("border-radius", typeof(ImageButton), nameof(BorderElement.CornerRadiusProperty))]
  [assembly: StyleProperty("border-width", typeof(IBorderElement), nameof(BorderElement.BorderWidthProperty))]
  [assembly: StyleProperty("color", typeof(IColorElement), nameof(ColorElement.ColorProperty), Inherited = true)]
  [assembly: StyleProperty("color", typeof(ITextElement), nameof(TextElement.TextColorProperty), Inherited = true)]