Retire sun.misc.GThreadHelper

sun.misc.GThreadHelper was introduced by JDK-7059886, for what looks like a co-ordination mechanism for initializing GTK on the platform. Initialization can be initiated by either one of the desktop module or Java FX.

sun.misc.GThreadHelper is not a critical API as defined by JEP 260, so should be moved out of sun.misc. It looks like it makes sense to move GThreadHelper to an internal package in the desktop module, where it can be access by both the desktop module and Java FX.