Introduce an ImgLib1-independent alternative to ShapeListImage

The 3D Viewer does not use ImgLib at all, the only reason why the
dependency was introduced was to support triangulation of TrakEM2-specific
data. Let's introduce an ImgLib-agnostic data structure to support that
use case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Support triangulating lists of areas

This is the API that should have been introduced in the first place,
rather than pollute 3D Viewer with a dependency it does not need.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Phase out ImgLib1

As we remove the corresponding public API (breaking backwards
compatibility), we have to bump the major version...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>