[Feature Request] `Slider`: Add step property that affect keyboard action, and foucs with mouse clicking

[asuper0]

trafficstars

When focus the slider and press left or right arrow, the value is change by the step, which is hard-coded to 1. This is not good when maximum - minimum != 100. So I suggest to add a step property, which determine the step of a key press action.

On the other hand, I found the only way to focus a slider is pressing Tab key multi times. It would be better to select it by mouse clicking.

[tronical]

That sounds like a good idea to me.