Fix Leak when closing devtoolsfx

[FlorianKirmaier]

This PR fixes and tests various memory-leak scenarios:

• Closing DevToolsFX, DevToolsFX should be collectable.
• Closing a Stage - DevToolsFX shouldn't create a leak on the original stage.

For both cases there is a regression tests.

[FlorianKirmaier]

Some Context,
I was working to host the new JPro FlexGanttFX Showcase as a demo, in cooperation with @dlemmermann
In the first version, I removed the devtoolsfx from the showcase.
But now it works actually quite well.

Demo: https://demos.jpro.one/flexganttfx-showcase.html (not announced yet)

So i hope everything is fine with the PR and it can be merged.

Another small note,
I think you could remove HostService from the API.
It's easy to access with new Application().getHostService.
Honestly, I've never seen this class used until now.
Didn't add it to the PR, to keep the context minimal.