RFR 8167636: jshell tool: Edit Pad should be in its own module

[Robert Field]

Remove the dependency of the jshell tool on the java.desktop module.
Uses modules and ServiceLoader so that Edit Pad is optional -- that is, 
a jlink'ed image can be produced without Edit Pad and without java.desktop

Bug:
     https://bugs.openjdk.java.net/browse/JDK-8167636

Webrev:
     http://cr.openjdk.java.net/~rfield/8167636v0.webrev/

Thanks,
Robert