DefaultLUTService: fix method order

Public methods come before private ones; see:
http://developer.imagej.net/coding-style#Ordering_of_code_blocks
LUTFileDragAndDropHandler: create new LUT display

At the moment, passing a ColorTable to the DisplayService's
createDisplay method does not work, but that will be remedied shortly.
So in anticipation of that, let's change the LUTFileDragAndDropHandler
to make use of it directly.

This eliminates the use of the LUTService's special-case createDisplay
method, in favor of the DisplayService's general-purpose method.
LUTService: change createDisplay to createDataset

The only functionality we really need here is the ability to wrap a
ColorTable in a Dataset that displays it as a ramp. We can then lean on
the DisplayService to wrap that Dataset in a display if needed.
DefaultImageDisplay: add support for ColorTables

The default image display can now display a ColorTable by wrapping it in
a Dataset using the LUTService#createDataset method.

This change also improves the code organization in the createDisplay
method to eliminate duplicate code blocks.
Merge branch 'lut-display'

This branch makes the Open command display LUTs properly,
fixing ticket #1406.

It works by enhancing ImageDisplay to support ColorTable
objects by wrapping them as Datasets.