Commented Issue: [WinRT] Support Windows 8.1 [336]

Fix binding issues on Win81 and add conventions for (some of) the new controls.
Comments: Already done: - add Visual Studio 2013 solution - add WinRT81 sample project - use default SettingsFlyout (instead of Callisto) in WinRT81 - use GetBindingExpression() in WinRT81 - add some more conventions for WinRT controls - use UpdateSourceTrigger in WinRT81 bindings