Feature/refactor slider

[drmacm]

This pull request replaces #42.

In addition to settings, this pull request includes refactoring of SleekCircularSlider and CurvePainter. The parameters for SleekCircularSlider are divided into:

• CircularSliderSettings (more info in the previous pull requests)
• CircularSliderValues (minimumValue, maximumValue, initialValue, angles and widths)
• CircularSliderCallbacks (3 callback functions)
• CircularSliderPainters (custom painters for background, shadow, progress bar and current value)

All the painters have uniform interface, accepting settings and values, so it's much easier to provide custom implementations for parts of the UI.

[matthewfx]

Hey,
I should have more time from the beginning of April. I left my day job and will fully focus on my ideas. I don't want your work to be in vain so could you please fix the animation for the programmatical value changes and the default properties when someone initializes the slider without any custom values (vide example no. 1). In addition to that, you would need to add the null safety now. Please let me know if could do it - otherwise, I might try to do it by myself. Thanks!