glTF export: Use plural folder names

Change-Id: If17628395ede512c00b666efc5511d9711edfb5a
(cherry picked from commit 9ee69cbd6f1143160aa96001462b933770cc6b76)
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.

(cherry picked from commit 4d8c4e7fe3e9b0ec6e14b5475a29d119e2023065)

Change-Id: Ib9b2044b1f36b32d980cb79f6dac8dcf94d6209b