[REQUEST] Easy Debugging

[kiraio-moe]

When debug mode enabled, a window containing variables to change the behaviour of Spine will be shown.
We can changed the values and save it as a JSON, overwriting the existing one. For safety, it will backup the original settings.json and maybe a reset button on the GUI.

The implementation will use RapidGUI as the UI library while save & load system for changed variables will use our own code. Initially will use PrefsGUI to handle it, but it's not supporting Unity 2020 and below.

Image taken from RapidGUI repository