ImgView: fix javadoc
WrappedImg: add missing license header
WrappedImg: add missing javadoc
WrappedImg: do not extend Img

Instead, WrappedImg is a "duck type" style interface that provides a
single method for unwrapping the Img in question.
Remove unnecessary ImgPlusView class

The following are equivalent:

    new ImgPlusView(raInterval, imgFactory, metadata);

with:

    new ImgPlus(new ImgView(raInterval, imgFactory), metadata);

It's slightly longer, but still one line of code. So the gain in
succinctness with ImgPlusView was minimal, and we decided not needed.
LabelingView: tweak javadoc