Add Abstract ColorViewBase for Shared Properties Between ColorPicker and ColorView

[robloo]

Is your feature request related to a problem? Please describe.

ColorPicker currently duplicates the entire ColorView's control template within it's own control template. The full reasons for this are outlined here:

#19663 (comment)

Describe the solution you'd like

@rabbitism Noted it should be possible to pull out the common properties into a ColorViewBase class.

    public abstract class ColorViewBase : TemplatedControl
    {
        // All ColorView property definitions
    }

    public class ColorView : ColorViewBase
    public class ColorPicker : ColorViewBase

Describe alternatives you've considered

The alternative is what we have today. Duplication in the XAML control templates and ColorPicker derives from ColorView.

Additional context

This probably needs to be done at the next major release due to the breaking changes in the control hierarchy.

[rabbitism]

I did a very simple prototype with copilot days ago,

https://github.com/irihitech/Avalonia/tree/copilot/simplify-colorpicker-colorview

I can see some issues with property change event handling duplicates. this should be carefully designed. others seems good.

[robloo]

That's a good investigation into this idea.

I can see some issues with property change event handling duplicates.

Well, if duplication still seems required it's probably fine to duplicate some of the property handling in code-behind for each control: ColorView and ColorPicker separately. I wouldn't overcomplicate this by putting code-behind in the abstract base control itself when the code needs to diverge. The abstract base would only contain the property definitions and code that is 100% the same in all derived controls.