The SymbolSources configuration is used to define new SymbolSources (providers of values for substitution symbols).
Contributions are of the form:
<source name="..." before="..." after="..." class="..." service-id="..."/>
Sources are ordering based on the name, before
and after elements. before and after
may be comma-seperated lists of other sources, may be the simple value
*, or may be omitted.
Only one of class and service-id attributes
should be specified. The former is the complete name of a class
(implementing the SymbolSource interface). The second is used to
contribute a service (which must also implement the interface).