RFR: JDK-8241840: Memoryleak: Closed focused Stages are not collected with Monocle

[Florian Kirmaier]

Hi everyone,

Here is my PR for the memory-leak I've reported some minutes ago.

PR: https://github.com/openjdk/jfx/pull/153
ticket: https://bugs.openjdk.java.net/browse/JDK-8241840

Greetings
Florian Kirmaier