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.

(cherry picked from commit 30cc85fd)