Improve the document diff UI in order to reuse it to show the differences between multiple documents

There are a couple of issues with the current document diff UI:

• it uses a lot of headings and explanatory text (labels) which makes it hard to read when there are many changes. It needs to be more compact.
• it uses both inline diff (for single line properties) and unified diff (for multi line properties), thus the appearance is not consistent. it should use only the unified diff.
• it doesn't have a summary (like a table of contents)
• it doesn't display detailed changes for xclasses
• it doesn't display detailed changes for attachments (XWIKI-9785)

On the technical side, the document diff UI is in changesdoc.vm and it cannot be reused unless we include the template as is. It would be cleaner to move the Velocity macros in a separate template and let the other UIs reuse whatever then need.

A propose regarding these issues has already been made by evalica at http://design.xwiki.org/xwiki/bin/view/Improvements/DiffHistory .