Description of Change

Refactors the lookup logic for the Xaml Navigation Extension to delay the lookup of the Target object and parent Page. Adds unit tests and exception logging.

Bugs Fixed

• fixes XAML NavigateTo on Partial Views error #1572

API Changes

API is unchanged.

Behavioral Changes

• XAML Navigation Extensions no longer return false if the parent Page cannot be found.
• The SourcePage is only set in the event it has not been specified by the developer
• In the event that the Parent Page is set as the Master of a MasterDetailPage, the SourcePage will be set to the MasterDetailPage
• A new virtual method has been added to Log exceptions. By default this uses the Xamarin.Forms Log which is the same mechanism used for Binding and other XF XAML Extensions

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard