Update examples for v1.89

[JamesWrigley]

Overview of important changes:

• Renamed ShowDemoWindow() to ShowJuliaDemoWindow() to avoid confusion with the official demo, and use it by default in examples/demo.jl.
• Use unsafe_load() where appropriate.
• ImDrawCornerFlags_* -> ImDrawFlags_RoundCorners*: https://github.com/ocornut/imgui/blob/docking/imgui.h#L3515
• AddBezierCurve() -> AddBezierCubic(): https://github.com/ocornut/imgui/blob/docking/imgui.h#L2927
• Use explicit key ranges in demo_misc.jl:
  • https://github.com/ocornut/imgui/blob/docking/imgui.h#L949
  • https://github.com/ocornut/imgui/blob/docking/imgui.h#L2122

Fixes #76 :crossed_fingers: Tested by opening all of the sections and playing around with them.

[Gnimuc]

Thanks! 😘