Fix duplicate database change event send on Library.add

[valrus]

Description

Fixes #5560. Also a couple other incidental changes / improvements:

• Add EventType that holds the actual string literals used for event sending. With type checking, this can prevent subtle bugs resulting from misspelled event names.
• Allow is_hidden to take str, and convert those str to Path. Without this fix, test_hidden.py was failing for me due to path.lstat() — test_osx_hidden passes a str, and str doesn't have a lstat method.

To Do

• Documentation.
• Changelog.
• Tests.