Add function to load Keyframes object for scening

[LightArrowsEXE]

Being able to call a function to load a Keyframes object (or even a filepath perhaps?) to use for scening would be a nice addition. This allows users who generate them for previewing to immediately load them without extra steps inside the vspreview window directly.

As an extra addition, maybe vspreview could also automatically check for keyframe files that match the filename. If this idea isn't veto'd, I propose it should check for a keyframe file in .vspreview/scening, and maybe make this an optional default path you can adjust in the vspreview settings window? This is pretty extra though, and not wholly necessary.