fix currentWindow keeping reference to old modalstage

[SKeeneCode]

fix #31

When openModal is called, the default parameter for owner is currentWindow which will look for a Window in the following order:

1. The components modalStage
2. The components root.scene.window
3. The primaryStage

The first time a modal window is opened the components currentWindow will return primaryStage and in the openModal function that component will have its root put into a newly built scene and that scene will become the scene of a newly made modalStage.

When the Modal Stage is closed the component has its modalstage set to null, but its root is still wrapped in a scene which is still the scene of that modal stage so next time openModal is called, the owner becomes the previous modal stage (because currentWindow will look at root.scene.window which points to the old modalstage).

This old modal stage is now the owner and because the owners position determines the starting position of the new modal stage it gets the placement wrong.

By removing the component as the root of the scene when closing the stage, it prevents that stage from being referenced by currentWindow in the future.

[the-rene]

Is there a reason, why this does not get merged?

As mentioned in #31, I can confirm, that this PR fixes the issue.

[edvin]

Thanks for this, and sorry about the late reply. Merged now :)