Separate Lectern and Book to match enchantment tables and its book

[NotStirred]

Previously a lectern redirected all pose information to its child book, which is different to the enchantment table. Likely because both lectern and book are entities

Main changes in this PR:

• blocks and entities can return multiple entities on toEntity and fromJson respectively
• lecterns are no longer poseable entities
• lecterns now return both the lectern and its book as entities

As a result, lecterns don't show up in the entities list (as they shouldn't), and lectern books show up in the entities list

[leMaik]

Lecterns are entities only because they are bigger than a block, i.e. they would be block entities. But then there's the book, which must be an entity (an actor) so that it can be posed.

Having a block entity and an entity at the same time would help, but I guess returning collections helps make this future-proof?

[NotStirred]

Yeah that was my thinking, if you'd like me to implement it differently let me know

[leMaik]

@NotStirred I rebased this on top of master, maybe you could verify that this still does what you intended. 🚀

As a follow-up PR, we could add a button to create a book entity from a lectern, right now adding a lectern creates an empty lectern and not one with a book.

[leMaik]

Should we rename the toEntity and isEntity methods to createEntities and hasEntities to make it more clear that a block may consist of multiple entities and that's why the methods return collections?

[NotStirred]

Yeah that's probably better 👍

[leMaik]

Rebased, renamed methods and added more javadoc.

Having Entity::fromJson return multiple entities doesn't make sense now that I think about it. Since we create an entity per entity we already store them separately in the scene file, so loading an entity from there will always only produce a single entity per entity.