Clean up GTTreeEntry

- Make -type public and delete GTTreeEntry+Private.h
- Make eligible methods properties
- Add documentation for those properties
- Rename -toObjectAndReturnError: to -object:
- Id -> instancetype
- @() instead of archaic -stringWithUTF8String:
Rename `-object:` to `GTObject`
s/strong/copy
Merge pull request #319 from PiersonBro/GTTreeEntry-Cleanup

Clean up GTTreeEntry