Virtual copies and edited indicator

[azacio]

In the software I mainly use, a virtual copy is a copy of a photo that appears on the film strip, but the actual raw file isn't duplicated. It's really handy for making alternate versions of a photo (e.g. a vertical version for social media) without having lots of extra files cluttering the folder.

Also, thumbnails on the film strip could have a little icon on them when they've been edited, as a visual indicator of what has been worked on.

[CyberTimon]

Virtual copies are now implemented and will appear in the next release: bc0797b

For the edited icon, see #203.

Thank you!