Taxonomy and book fixes applied to category

All relevant bug fixes that have been made to the book or taxonomy modules over the past month, have now also been applied to the categoryi module. This brings the category module in line with its core counterparts, which have more eyes looking over them, and are therefore more likely to have patches submitted for them.

This synchronization will be performed as regularly as possible (hopefully about once a month), in order to keep the category module compatible with its counterparts, as well as equally bug-free.