Major refactoring is needed in order to fix some issues

Lately, I've been working on BLOG-113: "Add the ability to display the panels of a blog outside of the blog context" and BLOG-16: "Cannot add a category that has the same name as a blog post" and while investigating the code I encountered several issues, that prevent these 2 issues to be resolved:

• the default blog doesn't have a dedicated location for categories
• the velocity macros are too inflexible to integrate the new work
• the code is not using the XWiki Model the nested pages are not correctly created everywhere.

The Blog documentation states that "Since 9.4 the multi-blog feature is supported" but this seems to be done partially as the blog panels can't display results for a given blog.

A migration script needs to be provided for this issue.