Display a few of a file group's members as labels underneath a file
group icon.
Add support for adding .java files to a buildml diagram.
Make sure that when multiple files are dragged across from the Eclipse
package explorer that they're added to the same file group.
Zero-sized file groups are no longer shown on the diagram, and are
garbage collected on exit.
Allow File Groups to be deleted, with "undo" and "redo" support.
Add undo/redo support for dragging file's from the Eclipse project
explorer onto a BuildML package diagram.