Adjust build.gradle mechanism for creating jalviewjs cores to better fit needs of developers/users

Currently build.gradle uses the following files to create closure-compiler cores:

utils/jalviewjs/_j2sclasslist.txt -> core_jalview.z.js
utils/jalviewjs/classlists/${name}.txt -> core${name}.z.js

(for all of utils/jalviewjs/classlists/*.txt)

There are two parts to this:
1) minor/easy: incorporate creating core_jalview.z.js into the generic utils/jalviewjs/classlist/${name}.txt mechanism, and have all cores named core_${name}.z.js
2) devise procedure or mechanism to create a suitable class list file for core_jalview.z.js