GitLab

write the metadata in the image as exif/iptc/xmp format
sync the image embedded data and the .comment metadata

[bug #575017]

allow to exit from fullscreen using the 'f' key
allow to rotate images with '[' and ']'

space to view the next image, backspace to view the previous image

When a sidebar is on the right side, it's hard to set its width
correctly.  Use a fixed size for now.

added map/unmap to the gth-image-viewer-tool interface

Also, renamed "Date" to "Comment Date", to distinguish it from the
file mtime, or various exif date tags.

added push/slide from bottom, cube

simplified the code as well