clarify and document behaviour of Jalview Develop and Jalview Test specific configuration files

The Jalview Develop application creates and (if it exists) preferentially reads its own custom properties file 'jalview_develop_properties'.

Nice to have but needs documenting - also would be good to be able to create a 'use_jalview_properties=true' property in the develop properties file so it preferentially works with the standard properties file, since that's useful for testing behaviour.