Added the option to strip out EXIF comments which have been generated by the camera by default when new images are added to the database.

I have two digital cameras. Both automatically add EXIF comments to each image, which is annoying to fix. My old Olympus C-470 adds "OLYMPUS DIGITAL CAMERA         ", my newer Canon IXUS 220 HS even adds pure whitespace: "                               ".

Such EXIF comments can now be defined and stripped out automatically when new images are read. Additionally, Descriptions consisting only of whitespace (probably entered by accident) are discarded on save now.