Enabling Glass thread checks

[Petr Pchelko]

Hello, OpenJFX.

FX is a single threaded UI toolkit. Glass (the underlying native window toolkit portability layer for FX) is being changed to ensure it is
accessed from the UI thread. You can follow progress in https://javafx-jira.kenai.com/browse/RT-26891

We are reenabling the Glass thread checks ones again. Previous attempts failed, because we've been finding some threading issues,
however now all the threading issues have been fixed and we are in a good state to switch on the checks again.

The following exception would mean that you've hit a thread check: IllegalStateException("This operation is permitted on the event thread only"). 
In that case please check your threading and if everything seems correct - please contact the Glass team.

Thank you.
With best regards. Petr.