Add alternative mine warefare symbols

This PR tries to resolve Issue #118. I have added the `AlternativeGraphic` attribute to the schema and updated `jmsml_D_Mine_warefare.xml` with references to the alternative symbols. I don't know if this is the correct or preferred way to resolve #118. I can easily change the name of the attribute and make other changes if you want to use the PR.

I have not tested how the changes affect the example application (I don't have a C# environment installed on my computer at the moment)