Java 9 illegal reflective access from DarculaLaf to HTMLEditorKit.DEFAULT_STYLES_KEY due to issue 41 in Darcula

@ham1 ([Bug 61884](https://bz.apache.org/bugzilla//show_bug.cgi?id=61884&redirect=false)):
When running with Java 9 I get the following warning:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by com.bulenkov.darcula.DarculaLaf (file:/home/td/coding/jmeter/lib/darcula.jar) to field javax.swing.text.html.HTMLEditorKit.DEFAULT_STYLES_KEY
WARNING: Please consider reporting this to the maintainers of com.bulenkov.darcula.DarculaLaf
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

Version: Nightly
Severity: normal
OS: Linux

Start by locating DarculaLaf and its access to javax.swing.text.html.HTMLEditorKit.DEFAULT_STYLES_KEY, then reproduce the warning on Java 9. Done means the illegal reflective-access warning is no longer emitted while JMeter starts and uses the Darcula look and feel.

Written by the indexing model from the issue text.