Added the option to strip out EXIF comments.

As Tobias writes:
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, ImageInfo::setDescription() now trims the description string.
Make exif commentsToStrip escaping simpler.
Fix compile error that snuck in in rev 968450a.
Highlight current index in Thumbnailview.

BUG: 254641