Allow specifying a gradle JDK in settings

[NepNep21]

This is helpful as the kls init script uses JAVA_HOME to determine what JDK to use for launching itself, so someone can't use that same variable to control gradle without ignoring the script altogether and redoing its logic elsewhere

[NepNep21]

would it be a good idea to make a separate PR to handle #392 ?

[fwcd]

Yes, that would probably be a good idea.