Add a function (or filter) to find all pictures without any tag

[SebastienThibaut]

Is your feature request related to a problem? Please describe.
It's not a problem but a missing function. It would be great if we can search pictures that don't have any tag.

Describe the solution you'd like
If possible, add a filter to find pictures without any tag.

Describe alternatives you've considered
I didn't find any solution to do that from the app.
Actually I check all my pictures date by date and add the tag. It's not really complicate but take a lot of time.