fixed gravity strength being possibly strong at level beginning
fixed restarting being allowed after getting the last ring
fixed save file's "played_for" being reset or set to another file's
fixed the displayed save file total play time not being updated on main menu
maybe there's other stuff
pause menu had classic issues of mouse inputs being eaten by something else (gravity)
restart didn't have working code anymore because of level logic changes
also remove the demo scene which I believe is unused beyond the jam version
