Enable cancelling localization sessions

[chrisfromwork]

This review addresses: #242

In this review:

1. ILocalizationSession is extended to have a Cancel function defined. This is beneficial because it eases cleanup when a network connection is lost.
2. SpatialLocalizationSession was created as a base class. This base class contains cancellation token sources for tearing down localization on cancel.
3. Dispatcher and DispatcherUnity were created to ease scheduling tasks on the ui thread.
4. We now lock when accessing the current localization session in SpatialCoordinateSystemManager.
5. SpatialCoordinateSystemManager now has a flag that states whether localization is running.
6. SpatialAlignmentVisual now displays to the user when localization is running.

Breaking Change Details:

Notes:

This review introduces breaking changes to ISpatialLocalizationSession. Contributors felt there was a need to be able to cancel sessions, which led to this change.

Migration Instructions:

1. Update custom implementations of ISpatialLocalizationSession to support a Cancel function.

[andreiborodin]

[stephenatwork]

Is there a reason to prefer a separate lock object? If this is only protecting currentLocalizationSession, would we lock that object directly instead?

[chrisfromwork]

locking on a null object throws exceptions. Since we want to populate currentLocalizationSession within the lock, it seems safer to lock on a separate, non-null object.