Made a common base class that implements INotifyPropertyChanged. Also implements a RaiseAndSetIfChanged call that streamlines property definitions. Some other various cleaning
- Moved NexusApiClient and all of its Dtos into their own package.
- Moved util methods into a new util class.
- NexusApi no longer creates new HttpClients for each request and uses one internal HttpClient instead.
- Added rudimentary tracking of rate limits.
