implemented support for "previewLink" type in the Location field, as
well as code to construct a url to a layer on an external ogp site (v1,
v2 or geoweb type), which is used when layer access is restricted at an
external institution.  The implementation required additional fields in
the repositories.properties file to specify the node type and the base
url.  Also created a mechanism for subclasses of the LayerRow object to
inherit events.