feat(scale): add on_release property

[Pikaguif]

Adds on_release property to the scale widget, a callable property that is called when any action the user does end (letting go of the slider, scrolling up/down) instead of any time the value changes.

Possible use cases are:

• Multimedia control, changing the position only once. (This is similar to how spotify does it, for example)
• Operations in which so many events causes a rate-limit

This is my first PR, if there's anything that is painfully wrong, I apologize in advance, and I'll try to fix it as soon as possible.