Group and ungroup figures

[Jagdaw]

As a JHotDraw user building a diagram, I want to combine several selected
figures into a single group so that I can move, copy, and arrange them as one
unit, and later ungroup them back into individual figures when I need to edit
them separately.
Acceptance criteria

• A user can select two or more figures on the canvas.
• A "Group" action combines the selected figures into a single group figure.
• A grouped figure can be moved, resized, and selected as one unit.
• A user can select a group and "Ungroup" it back into its individual members.
• The members keep their original attributes (colour, position, size) after ungrouping.
• Group and ungroup are both undoable (Ctrl+Z restores the previous state).
• The change is reflected immediately on the canvas.
• A grouped structure is preserved when saved as SVG (as a element) and reopened.