app: add config option to enable/disable layer-group previews
Preview generation for layer groups is more expensive than for other types of drawables, mostly since we can't currently generate layer-group previews asynchronously. Add a preferences option for enabling layer-group previews separately from the rest of the layer/channel previews; both of these options are enabled by default. This can be desirable regardless of performance considerations, since it makes layer groups easily distinguishable from ordinary layers.
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