Set of nice spinners for imgui

1. add a little code that enable test mode

#define IMSPINNER_DEMO
#include "../imspinner/imspinner.h"

int main(int, char**) {
  ...
  while (!done) {
     ...
     ImSpinner::demoSpinners();
  }
}

2. enable edit mode with checkbox 'show number' or play with options

Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.

MIT