Background layer refinements to consider

- Context menu "Copy to layer" -> ["New layer...", ...list of existing source layers]
- Remember the state of the "Copy current layer" checkbox in the "Add source layer" dialog
- Maintain layer when switching glyphs (as long as the target glyph _has_ a matching layer)
- Improve visual distinction between layer types ("editing", "other sources", "source layer")
- Allow visual distinction between layers (color, dash, custom?)
- Allow "deep editing" as long as the selected layers are compatible (deep editing is currently limited to _source main layers_ that are compatible)
- Show more clearly in the source layers list which layer is the "main" layer, say by styling the name bold
- Allow double-clicking on a source layer to _make_ it the main layer

_(To be edited with more items if needed)_