JMC-5368: Clear Stacktrace View when editor is closed

[Ken Dobson]

Here's a small patch that clears the stacktrace view when all JfrEditors
are closed rather than persisting the most recent stacktrace.

Bug ID: https://bugs.openjdk.java.net/browse/JMC-5368
Webrev: http://cr.openjdk.java.net/~kdobson/JMC-5368/webrev/

Thanks,

Ken Dobson