IdealGraphVisualizer bugfixes: * When removing graphs, ensure that the position indices in the range slider remain valid (although they might represent different graphs afterwards) * Instead of removing the last remaining graph of a group, remove the entire group. This prevents problems in some components that assume that a group always has at least one graph.

This is as a read-only copy of the mxtool (https://hg.kenai.com/hg/maxine~mxtool).
Having a copy makes the GraalVM repo more self-contained so that every revision
will always be built with a specific version of this tool, ensuring repeatability.

Changes to the tool should be made in a clone of the Kenai repo, pushed back
to Kenai and copied here.