v0.9.23

• Added accent color keys in ThemeKeys: SystemAccentColorKey, SystemAccentColorDark1Key, etc.
• Added CornerRadius for Thumb (use ui:ControlHelper.CornerRadius property)
• Fixed a crash when a checked ToggleButton is set disabled
• Fixed and improved elevation border brush
• Re-organized namespaces and simplified XAML resources (#22 by @ghost1372)
• Fixed some other bugs