ACT-2074 add support for custom identity links to Modeler

related to ACT-2038 - gui changes

added JSON conversion routines

NOTE: user must still extend the IdentityLinkType class and include the
relevant custom resource entries under the usertaskassignment element in
the stencilset.json file (eg. "customResources(Users/Groups):<name>").

       {
          "id" : "c4",
          "title" : "Custom Resource users",
          "value" : "customResourceUsers:administrator",
          "refToView" : ""
        }, {
          "id" : "c5",
          "title" : "Custom Resource groups",
          "value" : "customResourceGroups:administrator",
          "refToView" : ""
        }

Also updated StringUtils.isNotEmpty() calls to stringvalue.length() > 0
for better performance.