Xcode combines PNGs into a TIFF
add support for Trigger Events

Whenever a file's tags are changed, a notification is sent containing
the file.
improve the identifier for transient tags

Future-proofing: The identifier should contain enough information to
recreate the object from scratch.

The details string has also been updated to use + instead of , to more
clearly indicate that tags are being combined.
increase version to 2.1.0