add self-referencing properties

Add the ability to set self-referencing properties in the *.properties files of maven. This could be useful e.g. for javadoc plugin for a (hypothetical) javadoc.offlineUrl property, which points to where the offline javadocs are located:
project.properties could contain such locations which are part of the project, e.g.

javadoc.offlineUrl=apidoc/package1/apidoc/packages.html:apidoc/package2/apidoc/packages.html

$HOME/build.properties then should be able to extend the property by adding further offline URLs, e.g.

javadoc.offlineUrl=$

:/usr/share/j2sdk1.4.1/docs/api/

Of course, a general "self-referencing properties" could be useful for several other cases.

As far as I was able to test this feature, a self-referencing properties used like above causes a Exception.

I have no idea which code needs to be changed, so sorry, there's no patch attached.