RFR: 8228393: Add an indicator for external links in javadoc

[Hannes Wallnöfer]

Please review:

JBS: https://bugs.openjdk.java.net/browse/JDK-8228393
Webrev: http://cr.openjdk.java.net/~hannesw/8228393/webrev.00/

The external-link.svg is just added just for future editing as the CSS-inlined bits can’t be easily modified.

This is the smaller superscript version as shown in these examples:

http://cr.openjdk.java.net/~hannesw/8228393/demo/v3/IDN.html
http://cr.openjdk.java.net/~hannesw/8228393/demo/v3/SignatureMethod.html
http://cr.openjdk.java.net/~hannesw/8228393/demo/v3/package-use.html

Thanks,
Hannes