Update TreeController

-   "object" is now "content" in all the places.
-   The "path" observer in FRB enables a simplification.
Add "content" as alias of "object"

-   Migrate "objectAtCurrentIteration" to "contentAtCurrentIteration".
    Since this affects a lot of API surface area, I've elected to
    continue support for the former for the time being.
-   Migrate "currentIteration.object" to "currentIteration.content".
    Also, "object" is retained as an alias for migration.
-   Setting the "content" of a Reptition always replaces the content
    controller.  It struck me that replacing the content of an existing
    content controller is not a real use-case.
Fix paths spec