LT-22122: Word Export performance improvement (#397)

When adding an image, add it directly to the MasterFragment.
This has two benefits:
1. Avoids cloning the image every time the fragment is cloned.
2. Avoids the need to update the part ID in the Drawing.blip.Embed
   every time the image is cloned. Since the image is added
   directly to the MasterFragment the part ID doesn’t change.

Author: mark-sil