Allow to modify the section name of a section content after it has been created

Currently, when a section is being created, the section content sheets require the user to specify a section name before allowing the editing of the section content itself.

This mechanism has been put in place originally in order to avoid situations where users would end up modifying section contents and having multiple section contents of the same language point to the same section. However, in some use-cases, when refactoring unified documents, end-users may want to purposely modify the section name of a section content to a different value.

This issue aims at :

• Providing a configuration in the XWiki administration that would allow admins to enable / disable the capability for end users to modify the section name of a section content.
• Modifying the section content sheet in order to provide an input field to modify the section if the above option is enabled.