Don't allow dragging layers onto their own canvas

Carsten Hartenfelsrequested to merge
hartenfels/krita:feature/bug-512771 into master

Because it really only ever gets used accidentally by users being a bit sloppy about selecting or dragging layers, resulting in them duplicating layers unintentionally.

See for example https://krita-artists.org/t/i-keep-accidentally-creating-layer-copies/150650

I don't think this removes any useful functionality. All it did was serve as a weird way to duplicate layers and you can already do that with the duplicate layers button, so it's not like this increases the amount of clicks required to accomplish the action.

BUG:512771

Test Plan

  • Drag a layer from the layers dock onto its own canvas. Observe that it doesn't let you drop it there.
  • Drag it onto a different canvas. Observe that it works as before.

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  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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