Allow the list of figure types to be configured

Current, the types of a figure is static (i.e., figure/table) and defined in org.xwiki.contrib.numbered.content.headings.macro.FigureType

It can be interesting to allow admins to extend the list with their own figure types.

Two options:

• on xwiki.properties
• using new dedicated XObjects

In addition, CKEdit must continue to display a nice list of auto-suggested choices during macro edition.