Improve management for timed event graph

[Quentin Quadrat]

Using SemiHoward.teg:

• the arc T2 -> T2 is not displayed.
• arcs with cycles can be superposed
• Moving nodes not working.
• Left mouse click: shall not create places (which are not displayed)
• arc from/to dummy nodes shall create a new arc.
• Calling Howard: the cycle T2 -> T2 is not displayed. Transitions shall be red.
• Delete transition shall also removes hidden places
• TBD: Delete arcs shall also remove arc touching the removed place
• Menu: convert to timed event graph shall show invalid arcs
• Affichage: mettre un tokens + nombre, virer le nom de la place.
• Lors de la simulation, faire que les tokens soient toujours affichés sur leurs transitions finales