XDOM diff feature

This means implementing a component that computes a diff between two XDOMs and then store the result as a new XDOM (using Block parameters to indicate when Blocks are added/removed/modified).

The main goal is to be able to represent diffs that are not targeting HTML. For example be able to generate a PDF containing the diff when going through the LaTeX extension.

Note: This also means the following:

• adding support for this in the LaTeX Renderer to display nicely the changes
• modifying the HTML renderer to also display nicely the diff XDOM
• add support for the {{html}} macro in case it’s used in the content

Note: the XDOM diff also has some limitations: if the XDOM contains RawBlock (e.g. when the {{html}} macro is used it generates a RawBlock) or Verbatim Blocks then they’ll be considered as black boxes and the whole content would be marked as modified. This is not the case for the HTML diff. This is why we believe that the HTML diff is the best diff for the XWiki UI (in the Versions tab). Now the XDOM diff is interesting because it doesn’t limit us to HTML.

PS: we need a new jira component probably since it'll probably be done in a new maven module