Discuss design for ImGui wrappers

[karnkaul]

RAII, std::*, etc.

[karnkaul]

The approach used so far has these characteristics:

• RAII wrappers are (ideally) transient - should not need to store any state across frames, intended to be used in a single scope
• Dependencies are modelled via constructor arguments; eg NotClosed<Window> if an open window is required
• Users control as much of sizing / positioning as possible