Add hints for class properties in the class editor

Since the addition of the Vertical Forms Standard, we have been feeling the need to specify hints for class properties that are used in edit forms.

If we want to achieve this right now, we need to manually construct a translations key convention similar to "Space.Class_property.xHint=Hint" (as suggested by http://xwiki.markmail.org/thread/x52y3gfeolqsmfik).

What we should do instead, is to add a new property metadata called 'hint' that would serve 2 purposes:

• Act as class property documentation
• Act as property hint in vertical forms
• Act as form element hint in the object editor when editing a property

In this 'hint' metadata we should be able to enter a static string or an call to the message tool. An example of such a field is: XWIKI-7690

A couple of years ago, we used to have the tooltip metadata, but it was marked as unused 3 years ago and is now hidden by default: XWIKI-3635
While we could just re-enable this tooltip property, semantically it would not be completely compatible, since the original idea of tooltip was to show a tooltip to the user when the mouse hovered an input. This is no longer needed with the Vertical Forms Standard.

Note that the need for property hints is also present in ApplicationWithinMinutes so it is yet another reason to include it in the class editor.