[REQ] Use filename as the layer name when opening non-Affinity files

[przemoc]

If you open non-Affinity file, e.g. JPEG, the default layer name is "Background", which is inconvenient if you want to keep track of what you work with. It would be great if filename would be used as the layer name, by default or as an option. It would be also more consistent with existing behavior of drag and drop on the document, as then new layer name is the filename.