added pagination methods

[Robotv2]

No description provided.

[stefvanschie]

Currently the pages within a paginated pane don't have to be linear. That is, there might be non-existent pages in between pages. This causes some of your methods to not work correctly. However, I want to change the behaviour of paginated pane such that pages are always sequential in a future update. So I think it makes most sense to leave your PR for now and then I'll merge it after this change has been made.

[RedstoneFuture]

According to the documentation, negative pages are also possible at the moment:

"Note that the pages don't necessarily have to follow each other up; you can add a fifth page (index four) without having a fourth page (index three), or you can have negative pages, such as -1, -2, etc. however this is heavily discouraged."

For normal purposes, however, an "isFirstPage" and "isLastPage" method is very useful.