XWiki 7783 - Add hints for class properties in the class editor

I have added a new metadata called "hint" in the PropertyMetaClass.

Then, we can edit the content of the hint in the class editor.

I have also added a PropertyClass#getHint(), that:
- display the hint if the metadata exists
- fall back to a default translation key convention if not.

I hope you like it.

Louis-Marie