Update to RxUI 6.0.6 and Akavache 4.0

This long-overdue PR updates us to the latest versions of everything, and drops all of the crappy workarounds we had to do in RxUI 5.x series. Some changes:

* NuGet now works, so drop the `ext` folder
* PCLs now work, so we switch to a single core-library
* Xamarin.Forms exists, so let's set it up to work with the Core library
* AutoSuspendHelper isn't a mess, so let's set it up