Add a diff (changes) view option for "preview" mode

While writing XWIKI-17134, I realized that we suffer from the same problem in our current edit more, with the current "preview" button, as the one described in XWIKI-17134, i.e. it's very easy to override someone's previous changes, by mistake (e.g. copying entire content from a page on another wiki and pasting it in the new wiki's page content).

One very nice thing we could do to improve this and make it easy to allow the editor to quickly check if they are messing something up, without having to save and then compare changes with the previous version in an additional step, would be to have some kind of "View changes" button in preview mode that would (either in the current screen or open a link to a new screen) display the changes (using the changes/diff view and comparing with the current and/or previous versions of the document) that are about to be saved on the new version.

We could even display a small/quick status next to it (similar to the notifications bell with the red number of events next to it) mentioning the number of changed line the new version will introduce (when pressing save).

This way, the editor will be much more aware and feel more in control over his actions and their consequences, before actually saving and risking to mess something up (think event listeners, scripts, etc... things that might have immediate effect).