Develop a bi-directional TransformConnector

[dgeb]

This will allow for sources to ignore didTransform events raised by the other while transforms are being applied. This could eliminate the need for many sources to maintain caches.

Possible approaches:

• make two uni-directional connectors aware of each other
• create a separate bi-directional connector that encapsulates two uni-directional connectors
• create a completely separate bi-directional connector

[dgeb]

With the introduction of transformation tracking in #96, I don't believe that there is still a need for a bi-directional TransformConnector.