This converts all shared images with the prefix IMG_OBJS_DND from bmp to
png (as bmp is slowly being deprecated by Linux) and adds a 200% zoom
variant, in order to improve the quality on high DPI screens.

The ISharedImages constants for those images have been marked as
deprecated and for-removal. Instead, a new set of constants has been
defined, pointing to the set of images where source and mask have been
merged into a single file.