Fix crash when duplicating a live muscle plot

[adamkewley]

Happened during a presentation, unsure of exact repro steps but roughly:

• On MacOS (if that matters)
• Open arm26.osim
• Create a live muscle plot of a muscle that spans the elbow
• Duplicate it
• Segfault?

[adamkewley]

This was fixed but not tagged in a commit: the bug was (iirc) that the plot duplication routine was adding a panel to the PanelManager, which was holding onto Panels in a std::vector, which reallocated due to the addition, which invalidated a pointer further up the UI stack (e.g. it->onDraw()) etc. etc. - mmmmm native development