Feature/change: keep a node that died in the simulation until manually removed

[EskoDijk]

If a node crashes/dies, currently it will be auto-deleted from the simulation. This has some unwanted side effects:

• user confusion (where did it go / why did it go away)
• little visual feedback on the crash
• risk of overwriting node's logs, when starting a new node with same number
• with automated scripting: risk of getting different node ID's during different runs if sometimes nodes crash. + risk of losing valuable crash output.

This item is to consider a different approach:

• keep the crashed node in the simulation, in a separate/different state ('dead' ?)
• in the GUI, show it as grey with the X symbol ('failed') on it.
• user can manually remove the node e.g. after inspecting logs.

[gmarcosb]

I think removing it if it crashes/dies is totally reasonable

Could maybe have an option to keep-alive, possibly set through OTNS; but I'd put that as super-low priority, what you have currently works beautifully