Improves behavior when multiple attempts are made to remove one or all ViewModels

[michaelpduda]

1. In the samples, adds a confirmation popup to the shared lists for manual testing.
2. Re-arranges the test project namespaces and folders.
3. Prevents the UpbeatStack from attempting to remove a ViewModel more than one time at once when it has called RegisterCloseCallback. Now, the first attempt will execute and any subsequent attempts while the close callback is still running will be ignored.
4. Improves the CanExecute behavior for the RemoveTopViewModelCommand by also checking if that ViewModel is currently running a close callback.
5. When attempting to close all ViewModels, will now wait for any already running close callbacks to complete.