Add message context for scene types ‘Portrait’ and ‘Landscape’

In the Exif metadata, the ‘Portrait’ and ‘Landscapes’ strings
mean actual photos of people and landscape, but in the rest of
the Krita UI, they mean vertical/horizontal paper orientation.
This adds ‘msgctxt’ info for the Exif strings, so that they can
translated differently than the paper orientation strings.

BUG: 410342