Add rating stars and deletion candidate buttons

[lucidtanooki]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

Hi everyone,
I use Gallery a lot, since before the fork. There is one feature I've wished for a long time now and I am willing to help, maybe with a PR. But only if it is wanted of course.

Feature description

Add a feature to rate images:

• show a rating button along with the other buttons (like favorite, delete, edit, share - I know those can be enabled in the settings)
• the button opens a dropdown: "deletion candidate", 0, 1, 2, 3, 4, 5
• add a settings to store the rating either in Exif "Rating" or in local database like "Favorite"
• add a filter to only show images with a given rating (might be a second feature)

Best regards,
lucidtanooki

Why do you want this feature?

• easier deletion of multiple photos
• do some work on the go without a full fledged suite like Darktable
• (for exif mode: make your work persistent)

Additional information

What to take into account?

• exif or local database?
• full -1 to 5 (marked for deletion up to 5 stars) or simpler (delete, none, favorite)?