Back-port Activation and routing fixes to Reactive UI 5.4

This PR replaces #506 (mostly here because I just realised I deleted my fork of RXUI which the original PR was from... :-1:) and contains various features that would be good to back-port to 5.4 from 6:

## Activation

Back-ports the new activation bits as described in #503. Not only super-useful for things like cancelling unnecessary HTTP requests, but also fixes some nasty memory leaks when used with WPF. Also includes the fixes in #526 to only run the activation when views are hit test visible.

## ViewHost Templatability

As per #532, allows ViewModelViewHost and RoutedViewHost to have a template which does not include the previous content site, which can be used for fixing airspace issues in WPF.