Fix PHPDoc for Layout class and related code, fix return types

[alies-dev]

There is only one arguable change: fireRemoveCallback() method return type (it's declared as mixed) is unused in the package. I propose to remove it.

I also found a weird situation with Layout::$limit, not sure where it should be set, but I fixed PHPDoc for the constructor (it contained $limit that was removed by https://github.com/whitecube/nova-flexible-content/commit/ca0b6e53f834948a4f7f79aa714d8ee721e3fe3e), also one Layout object initiation was using this removed parameter (I removed it) and fixed duplicateAndHydrate method where I think the value of this property should be copied/replicated.