Implement interceptDragAndDropFile

This allows us to intercept drag-and-drop file requests and pass them to
SCIFIO.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Do not intercept dragged 'n dropped LUTs

Otherwise, ImageJ 1.x will not apply them properly.

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