thumbnails(): stop using return value from Set.call()
code.js: add trace() method
- File_Set(): fix set key
- watch():
  - implement watchers as an instance attribute instead of a value attribute
  - transactions support implemented using _add_value() and _remove_value() instead of _add() and _remove()
  - _add_value(): test if file is already added
- watch_directory():
  - on_change(): implement a shared timeout to group a number of changes in a single transaction
  - stat: add inode
  - stat: in debug mode display all modified properties
- test/http.js: add trace for watch_directory test, comment-out 10 seconds updates
- Update documentation