ImGui Support Extensions

[Robadob]

Following #100 there are several possible low priority extensions which could be added should the need arise:

• Allowing users to change the colour scheme (currently it's fixed to ImGui::Dark).
• Allowing users to customise the format string for environment property printing (%g is used for floating-point, but this can create unwanted high precision numbers in rare cases)
• Allowing users to manually position panels within the window (they can move them after sim has started)
• Allowing users to mark panels begin collapsed.
• The random seed may potentially be rendered incorrectly for the first frame