Add accelerating/decelerrating sliders feature

[hi-mei]

For now, its is clear that some songs have a very unique sound design which can be visualized not just with shape, but also with a changing speed of slider ball thru the same slider.

Currently, it is allowed in RC to use things like this:

Map in question, the timing is 03:56:521 - if you want to check what it is. Stuff like that improves the player experience by a lot.

So the issue with that is that you have to put around 2-3 hours to make such slider good just for a basic shape.
I think that would be amazing to have such tool to slow dows/increase the slider ball speed thru some points of the slider.