Revert "Make glTF support an experimental feature"

There are others who think this will work better before the
release.

This reverts commit 653bac184f579524bc25a9f40281763a736ae6fd.

Conflicts:
	sd/qa/unit/import-tests.cxx
	xmloff/source/draw/ximpshap.cxx

Change-Id: Ie8747745a9a89ce4d6500a35d15a98e28bda840a