Sandbox autosave

[xen-0sd]

While the player is in sandbox mode, the engine would maintain an autosave.json file in the form of

    {
        "shape": VARIABLE_SHAPE,
        "x": VARIABLE_X,
        "y": VARIABLE_Y,
        "scale": VARIABLE_SCALE,
        "color": VARIABLE_COLOR
    },

where the variables are written to the file live as the player moves the shape around.

This would double-fix #31 and ensure not an ounce of progress is lost if the game crashes.

[SuperFromND]

I'm not opposed to the idea per-se, but so far we only save files to disk if the user explicitly triggers it (exporting in Sandbox, saving configs, opting in to logfiles via -l, etc etc). I think adding the confirmation dialog box is a good enough solution for preventing accidental creation loss for now, but I might warm up to autosaving later.

[SalsaGal]

Maybe this could go into the system's temporary folder?