[Feature Request] Methods to be added to `Graph` API.

[bheijden]

🚀 Feature

• Graph._remove_engine(): Should replace all engine node graphs with the agnostic object definition and remove the added engine.
• Graph._agnostic_object(name="obj"): Should replace the engine node graph with the agnostic object definitions.
• Add engine to EngineGraph & automatically connect tick if EngineNode is added.
• Add legend to dag graphs.
• Add engine: EngineSpec as argument object engine methods.
• Check that arguments of EngineSpec.add_object() are the same for Engine.add_object().
• Per default, use the straight line style as it is less cluttered. Make linestyle an argument to the gui render function.
• Visualize all registered EngineStates after an Object has been substituted by its EngineGraph.

Pitch

A clear and concise description of what you want to happen.

Checklist

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