Add docs for defining "events" on viewables

[PhilippMDoerner]

Based on this discord discussion

It appears this particular feature is not yet well documented and could use an example in the book.

Could be written as an additional section in the Custom Widget docs.

[can-lehmann]

This might also work well as a "Viewables" guide (https://github.com/can-lehmann/owlkettle/issues/153).