Add parameter to open nodes of DocumentTree to given level/depth

By default, the tree generated by the documentTree macro will only display the nodes on the first level of the tree. All child nodes are collapse and must be opened by hand by the visitor of the wiki if they want to see them.

The 'openTo' parameter of the documentTree macro (and the tree macro) allows the user to open/expand just a single branch of the tree. It would be nice to have more options here.

Introducing a parameter 'openToLevel' could introduce the following behavior:

• openToLevel=1 would yield the current behavior (only top-level nodes are visible at first).
• openToLevel=2 would expand the top-level nodes, such that they and all their children are listed.
• openToLevel=3 would list top-level nodes, their children & grandchildren (i.e. nodes of the first two levels are expanded).