Rearranging for Xamarin.*

This PR rearranges all of the UI Toolkit libraries (`ReactiveUI.Xaml`, `ReactiveUI.Cocoa`, etc ) under a shared root directory of `ReactiveUI.Platforms` (the Assemblies won't change names, just the file locations). 

This also creates a new Solution file named `ReactiveUI_AllVS.sln`, which attempts to build everything that Visual Studio can currently build into one Solution (VS can't build Xamarin.Mac at the moment), and renames `ReactiveUI_Mono.sln` into `ReactiveUI_XS.sln`, which *only* builds things that can't be built in Visual Studio