Add an option to merge the space home page node with the space node in the document tree

When the document tree macro is configured to show the spaces, the space home page node is a bit redundant. Would be nice to have an option to merge the space home page node with the space node.

Note that as a consequence of the merge the nodes that hold the translations, attachments and objects will have to be displayed under the space node, and since the space cannot have such entities, the solution is to display the space as a document. This will give the user the impression that the wiki has nested documents.