Add a 'showRoot' parameter to the tree widget to display the current root

Currently, we have a root parameter that specified what is the root of the tree, but the root itself is not displayed in the tree by default and only top level node are displayed.

It would be very useful to be able to also display the currently specified root since, in some applications, it might be a requirement to be able to select the root itself as target for a particular operation.

This showRoot (or whatever we want to call it) option should probably also be made available in the macros (tree macro, document tree macro) that use the tree widget.