Allow decimal degrees angle in rotation popover

[OliBomby]

Allows you to make patterns with precise angles and geometry, like a spiral that rotates 22.5 degrees for every circle.

I followed the suggestion by @bdach ppy/osu-framework#6376 (comment) to use a double bindable setup which bidirectionally linked with value change callbacks.

I let the Current bindable also change the SliderCurrent bindable so when you create a slider with textbox input with the old api it still functions the same way.

[bdach]

The API design on this is unacceptable. Current and SliderCurrent cannot coexist as public.

If I were to suggest an alternative, this should be a

public T? SliderPrecision { get; set; }

or similar. And it should throw errors when attempting to set a value that isn't a multiple of Current.Precision too.

[OliBomby]

Sure. Implementation is a bit weird though since you can't access the precision of the slider because it only exposes a Bindable instead of a BindableNumber.