Dependency Checker Collections

[KevinJump]

Move dependency checkers into a collection, so we can have decouple the dependency checkers from the SyncHandlers and have multiple dependency checkers loaded for each item type.

This will allow us to add dependency checkers for new things like relations, public access etc without having to alter the single dependency checker in the core, and not have to tie them to the main release.

[KevinJump]

In #104