RxApp refactorings.

This branch is for refactoring to RxApp. 

Currently this pull request moves a whole bunch of stuff away from RxApp such as:
- DI, now with IDependencyResolver.
- Unit-test detection.
- Design-mode detection.
- MessageBus nog longer on RxApp.

## TODO:

- [x] Rename `FuncDependencyResolver` to something better, create a delegate-based implementation of IDependencyResolver which just takes Funcs for GetService and GetAllServices.
- [x] Possibly change how Unit Test detection is working, as the current implementation assumes you're referencing Rx-Testing which might not be true