Fix duplicate shortcut key configuration items in Keyboard shortcuts Window (fix #4387)

[Gasparoken]

This PR is a follow up to https://github.com/aseprite/aseprite/pull/4453

Is this new approach suitable @dacap?

I think Open Sprite : can be improved but I need your preference on the matter.

I can squash the 4 commits and then push-force if it's better for you.

fix #4387