Allow shortcut setting in duplicate and snapshots

[Phemisters]

There were a few buttons which could not have shortcuts set, and this PR attempts to fix that.

This follows a pixls.us discussion, where a solution for the side-by-side button in snapshots.c was suggested.

I don't see anyone else attempting this, so I've tried the suggestion (which works for me) and created this PR.

While working on it, I noticed two buttons in the darkroom duplicate manager module also appeared to have no shortcut setting ability. This appears to be a different type of button set-up, so I have attempted to copy what I see in other places (using self instead of NULL). This seems to work when I tested it, but may be the wrong approach. I can easily remove the duplicate.c change.

[dterrahe]

Both changes looking good. Thanks for fixing the oversight!

[TurboGit]

We have a new string here, so targeting 5.6.

[Phemisters]

I see this has the release notes: pending label. Since another PR (#20235) also enables shortcuts I think they could be covered by the same release notes entry, assuming both PRs get merged. I'm not sure how release notes are handled, but I think something like this might be enough:

Enabled shortcuts for some existing buttons in duplicate manager, snapshots, and AgX modules.

[TurboGit]

Thanks!