Support property references in XML files

[rougeSE]

When deploying Worldwind within isolated environments with isloated WMS servers, there is no way to centralize the definition of the server name, instead, each of the layer XML files have to be updated. This change allows any of the WW XML file to contain a property reference, ${property.name}, and have these properties resolved when the text is retrieved. Properties can be defined in three different ways; 1)worldwind.xml <Property>, 2) Java system property (-Dproperty.name), or 3) using the legacy WW properties file.

Testing: UnitTest for WWUtilTest was updated to Perform testing of property reference resolution from Configuration and system properties. Test included nested references and logger generation for missing properties.

[zglueck]

@rougeSE thanks for the contribution. The team is heavily focused on some other tasks right now so it may take a bit of time to review your pull request. We do appreciate your willingness to share code!