A small feature and bug fix release to clean out some of the backlog before 4.0.0.

Included are the following

• #439 Fixes up an issue using commas in constant values cm:Message.Attach="ShowTitle('One, Two, Three')"
• #432 Stop calling InitializeCompontent on Xamarin.Forms, this was mostly used for a underused feature of .xaml files without a code behind. On the non-XF platforms this method has re-entry protection, the XF version did not.
• #423 Fixes a null reference in the Setup.WPF sample.
• #417 Handle a ContentPropertyAttribute that doesn't supply a property name (WPF's Frame does this).
• #364 Support OnViewLoaded in Xamarin.Forms when the view is a ContentPage.
• #352 Add an extension point BindingScope.GetVisualParent to help people who use visual "bridge" libraries.
• #319 Add $clickedItem Special Value for WinRT / UWP based projects.
• #278 Add a navigation service for WPF and Silverlight with a sample. Note this service may change in 4.0.0 with an effort to align all the navigation services.
• #247 Add an extension point on PropertyChangedBase and BindableCollection<T> named OnUIThread to allow people to customize how property changed notifications are dispatched to the UI thread.