Directly referencing webapp parts from within module parts

[MartinLeidig]

If you want to include a part located in the webapp dir from within a module include part, you have to use dynamic search. There should be an opportunity to directly reference the part without having to do a search, e.g. setting module="NONE"

[MartinLeidig]

Added support for special names: "WEBAPP" for referencing webapp parts and "PAGEDEF" for referencing parts of the module containing the page definition.