glTF export: Use plural folder names

Change-Id: If17628395ede512c00b666efc5511d9711edfb5a
glTF import/export: fallback must be the second one inside the frame

The previous solution was good because older LO versions
import the fallback image without changing their code, but
it came out it does not fit to ODF standard so export
fallback image as the second object after the glTF model.

Change-Id: Ib9b2044b1f36b32d980cb79f6dac8dcf94d6209b