Add exif tags to systemtags table

[JonaLoeffler]

This PR contains a prototype for #891

I know this implementation might have a few issues, I just wanted to get the ball rolling and understand how one would even start to work on this feature.

I am actually more interested in supporting Digikam's face tags, which are stored as Exif tags as well.

[JonaLoeffler]

Maybe this should be behind a settings flag to be disabled by default? I can imagine this will be surprising to many users if enabled by default, but the option will be appreciated by others.

[NJugel]

Maybe this should be behind a settings flag to be disabled by default? I can imagine this will be surprising to many users if enabled by default, but the option will be appreciated by others.

Wow, thanks for the first step. That looks very promising. Unfortunately, I'm not a developer in the community, but I'm happy to see the first real progress.
Maybe the flag can be set in the import popup.

[pktiuk]

I think, that a good to merge, and doable POC would be displaying a simple list of tags in the image info panel

For the sake of simplicity it would be read-only, but it would be a good start.