Add export for depths of images taken

This adds an option to the export dialog to write an ascii file containing
filenames of image files a tab and the depth at the time of the image
taken.

[Dirk Hohndel: whitespace cleanup]

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>