Replace NullableColorPicker ContentDialog with Flyout

[carlos-zamora]

Summary of the Pull Request

Updates the NullableColorPicker to use a flyout instead of a content dialog. Frankly, it should've been this way from the start.

#19561 is an issue regarding the rectangle on the right side of the picker. The complaint being that it should be something more useful than a preview, an idea being that it could be a lightness gradient. Unfortunately, the WinUI color picker doesn't let you do that. It's just a plain preview.

That said, there's a lot of customizations that can be added still to increase value here. To name a few:

• IsColorSliderVisible --> a color slider to adjust the lightness of the color (as desired in Color Picker Lightness Bar/Selector Inoperative #19561)
• IsHexInputVisible --> an input field to see and adjust the hex value directly
• IsColorChannelTextInputVisible --> several input fields to adjust individual RGB channels or switch over to HSV

However, the content dialog doesn't allow for text input due to a WinUI bug and it's too small to display all of those controls.

Instead, I just discarded the content dialog altogether and opted into a flyout. This makes it a more consistent experience with the other color pickers (i.e. tab color, edit color scheme page). This also adds space for all of the functionality mentioned above (those properties are enabled by default).

Validation Steps Performed

✅ selecting a color still works

Closes #19561

[carlos-zamora]

Demo