This removes the MessageBoxOptions input to MessageBoxViewModel, and
replaces it with explicit FlowDirection and TextAlignment inputs.
I think this is fair enough - we weren't using most of the members
of MessageBoxOptions, so allowing them to be provided was misleading.
Fixes#12
Having it as an extension method is good architecturally, but make it harder
to change its behaviour. This way, users can override it easily if they wish.
This also means that the WindowManager does not depend on the IoC service
locator, insteady accepting a Func<IMessageBoxViewModel>