Refactor the way ${X}matter is handled

[ysb33r]

The logic needs to be change to work as follows.

• If a level one [dedication] is found and it is the first, it will become a dedication page rendered before all other parts of the book.
• Level one [preface] is generated after [dedication]
• Each level zero become a part and only the partinto blocks will be processed.
• Each chapter is added into a tree per part.
• As soon as any backmatter level one is detected, al further parts with introductions will be ignored