Make configurable the maximum number of child nodes to display on the Navigation Panel

Currently the The maximum number of child nodes to display when expanding a parent node for the first time on the Navigation Panel is set to 15 by default, see "limit" parameter on the Document Tree Macro Documentation.

The objective of this ticket is to provide a possibility to easily update the default value of the "limit" parameter (ex : under the "Look & Feel > Navigation Panel" section on the wiki administration)