#398 Clean up and fix Model property for Xamarin (#599)
#398 Clean up and fix Model property for Xamarin (#599)

Clean up the code for the Model property so it is easier to read amongst
all the compiler directives. Also raise PropertyChanged when the
property is changed in the Xamarin world.
BrettJaner/csla,ronnymgm/csla-light,jonnybee/csla,JasonBock/csla,MarimerLLC/csla,MarimerLLC/csla,rockfordlhotka/csla,rockfordlhotka/csla,ronnymgm/csla-light,ronnymgm/csla-light,BrettJaner/csla,jonnybee/csla,JasonBock/csla,jonnybee/csla,rockfordlhotka/csla,JasonBock/csla,MarimerLLC/csla,BrettJaner/csla