Add display param to Colorbar formatter. [1660]

This parameter name may prove to be temporary. The _Layer object being
passed with the display parameter must (for now) have an ID. This can
be specified like so:

layer = disp.getLayers()[1]
layer.getJavaInstance().setId('foobar')
print layer.getJavaInstance().getId()

This requirement is kinda clunky, so expect change!