Enable the use of master pages in JessyInk

Master pages are an often requested feature. Inkscape supports clones, which can be used to achieve a similar effect. However, the element in question, still has to be copied on every slide. It should be possible to change the JessyInk code in such a way that a layer with a particular name (e.g. "Master") is deleted from the normal tree and than copied as a child of each layer.