ij-legacy: let MacAdapter do its thing in legacy mode

This fixes drag 'n drop onto the Fiji icon in MacOSX' dock.

Reported by Curtis Rueden.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'legacy-macadapter'

This fixes Fiji's regression on MacOSX where dropping an image onto the
Fiji icon in the dock would not open the image.

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