Prevent storing multiple copies of the same book

[ShnitzelX2]

Summary

Bugfixes "prevent storing multiple copies of the same book"

Purpose of change

Fixes #73282

Describe the solution

• Adds a function that removes duplicate items from an inventory_selector, discriminating by itype and optionally by variant
• Adds some basic docs to inventory_selector and related classes
• Adds an is_identifiable() function; this is currently just is_book(), but it allows a check for doing the identification so that non-books can be scanned in the future

Describe alternatives you've considered

Testing

Spawned textbooks 100 times and used "Store books" option with smartphone, noted no additional copies in list, but variants were still present.

Additional context