Improve process of building the book content #14

Content building

Building the book could be improved. Currently almost everything is done client side on first open and is cached afterwards indefinitely.
This has the drawback that e.g. block links can only be displayed after first opening the book, because the links are only registered during build. Also the content consumes memory.
One option may be to build the structure (categories and entries) during world load and only build the pages on demand (and do not cache them).