Tree: Make parent and hasChildren properties configurable

While one could argue Tree is a simple reference implementation, it is perfectly usable as a mixin for hierarchical data when the parent-child relationship is tied by parent IDs.

Allowing the `parent` and `hasChildren` properties to be customized gives this even more mileage, rather than forcing services or client-side code to massage them to those specific names to work with this mixin.

Includes updates to Tree tests.