One issue that especially newer users often run into is that they accidentally reset changes to the scene by switching frame without setting a keyframe first.
Therefore, this patch adds a new color that is used to draw the button if the current value differs from the one that would be set when switching to this frame.
This works both for existing keyframes as well as for currently interpolated frames.
Unfortunately the flags in but->flag are full, so I had to move the new flag to but->drawflag and pass that to all relevant functions.
I went with orange for the color for now since afaics it fits with the green and yellow that are currently used for keyframe states and since it's somewhat reddish to signify that there might be something to look out for here.
I hope I got the right reviewers, please add/remove yourself/others if I got someone wrong.