[Linux] No more drag events when new Stage is created in drag handler

The issue exists in all versions since java8, and only on Linux (Windows and Mac OS X are fine).
It is a blocker for any docking framework based on dragging an undecorated stage as opposed to drag-and-drop, see

https://github.com/andy-goryachev/FxDock/issues/6
https://github.com/kasemir/FxDocking/issues/1

To reproduce:
- launch and try dragging the red label around

Expected results:
- an undecorated translucent stage is created and is dragged around

Actual results:
- the stage gets stuck and cannot be dragged