Identifies the entry, for ISheetLayersRegistry.unregister. Case-insensitive.
The layers: one grid child or an array of them, each rooted in a Layer and each with its own
key, as in a sheet's layers.
OptionalsheetsThe sheets whose surfaces get the layers. Omitted, every sheet's do.
OptionalsurfaceThe surfaces the layers are rendered on. A layer on more than one reads which it is on with useGridSurface.
Layers a plugin adds to sheets, through DefaultSheetLayersRegistry.