Relax ImGui C++ version requirement

clang-format code

Move mouse click events to global struct

- ImGui::IsMouseClicked, ImGui::IsMouseReleased, etc. were getting called
  in many different places. Caching the events in one place makes it a bit
  easier to keep track of, in case we want to make mouse inputs customizable,
  for instance.

Add multi-editor example

- Side-by-side node editors seem to work! 🎉

Update README.md