Adapt the management of the blog categories to the multi-blog feature

This task is part of the blog application improvement proposal: http://design.xwiki.org/xwiki/bin/view/Proposal/BlogImprovemultiblogsupport

This task will cover the following aspects:

• Implement the categoriesSheet page to be used as a sheet to manage the categories of blog
• Adapt the Blog.Categories to the multi-blog feature
• Adapt the Blog.ManageCategories to the multi-blog feature
• Adapt the blog categories management inside the blog post form to the multi-blog feature