Improve server port offset detection by inspecting configured system properties

The default Wildfly standalone.xml config has port offset configured like this:

<socket-binding-group ... port-offset="${jboss.socket.binding.port-offset:0}">

With the way our configuration process is set up we don't change that. Instead we add a system property to the same config file:

<system-properties>
    <property name="jboss.socket.binding.port-offset" value="10"/>
</system-properties>

The problem is JBoss Tools doesn't detect this currently. It could be improved to inspect the port-offset attribute and find the corresponding system property value.