Merge Layer Down/Merge Layer Up

[mateuszmigas]

Is your feature request related to a problem? Please describe. Currently, there is no way to merge layers. All the core mechanisms are in place, so it's just a matter of using them to implement this feature.

Describe the solution you'd like

1. Add new commands and canvas actions:
   • "Merge Layer Down"
   • "Merge Layer Up"
2. Incorporate these commands into the Layers panel.
3. Ensure that these commands are visible in the command palette.
4. Utilize the Image Processor for merging layers in canvas actions ("fromMergedCompressed").
5. Confirm that undo/redo functions properly.
6. Examine Paint.net to observe how this feature operates in practice.

Describe alternatives you've considered N/A

Additional context N/A

[mateuszmigas]

I am only adding "Merge Layer Down" because that's what other tools do.

Icon: https://lucide.dev/icons/merge