Intercept image opening also in File>Image Sequence...

Adam Webb reported that image sequences of LZW-compressed images
could not be read, even if opening the images individually worked
quite well.

Let's intercept that code path, too, to let SCIFIO (or whoever registers
a legacy opener) handle those images.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'open-image-sequence'

Support intercepting opening images even when loading image sequences.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>