[Feature Request] Add garbage collection sign to the released memory blocks

[MarkChenYutian]

Is your feature request related to a problem? Please describe.

Not really, but in 15-122, student draw a Pacman-like icon near a memory block that is no longer accessible by the program to indicate that the block will be collected by the garbage collector. It would be nice if C0 visualizer can also follow this behavior / rule.

Describe the solution you'd like

When a memory block is no longer accessible for the program (detected by the BFS on heap as currently graphical debugger do), instead of hiding the memory block, show a "grey out" version of memory block floating around in the graphical debugger view.

1. When student click on it, remove the node (garbage is collected).
2. If the node is floated around for more than N (maybe configurable?) steps, delete the node automatically.

Describe alternatives you've considered

N/A

Additional context

This feature request is from prof. Iliano