Allow $entry->getNextSibling() and $entry->getPrevSibling() to work in Matrix fields

[dlindberg]

Currently $entry->getNextSibling() and $entry->getPrevSibling() are only available in sections that are structures. Because matrix fields have a specified sort order, it would be helpful if these functions could also be used in that context as well. For example, if you wanted to be able to navigate between the blocks, either with a slider or paginated. It would also be nice if you could do that in order for a relations field, but that seems a bit more complicated.