Introduce an easy way to enable heading numbering from the UI

There should be an easy way (select/checkbox) to enable/disable heading numbering. Ideally, this should be available during editing. The obvious choice is to put a new property into the information tab, but this has two issues:

• We need two new UIXPs to be able to add them
• There is currently no proper support for editing not-yet-existing objects, in particular for editable properties and during preview

Alternatively, this could be implemented based on https://jira.xwiki.org/browse/XWIKI-12497