Use known LUTs as defaults

When populating the default LUTs for a DefaultDatasetView, we have an
ImgPlus available which potentially has attached color tables. We should
use these ColorTables if they are available, instead of always using
default tables.
Be less aggressive when defaulting color tables

If channel == 1 there may still be a color table attached to an ImgPlus
to use.

Instead of defaulting to grayscale right away, we should check the
ImgPlus. If nothing's found, then we can default to grayscale or color
as appropriate.