Remove path throw in the LayerTree plugin

and add some doc.

fix #705

I would rather write:

In ``CGXP/plugins/LayerTree.js`` (``ptype: "cgxp_layertree"``),
parameters ``themes``, ``uniqueTheme``, ``permalinkThemes``,
 ``defaultThemes``, ``wmsURL`` have been moved to the output
widget. As a result they must be encapsulated into an ``outputConfig``
object in the plugin configuration.