Reuse colour picker code

With the ColorDialog coming soon to QtQuick.Dialogs, we should take the opportunity to move the colour pickers out from the non-native implementations and into reusable components that users (including Qt Design Studio) can use in their code. We could do this in two ways (names are just placeholders):

1. Super simple e.g. HslColorWheel, HsvColorRectangle, etc. types, which are purely visual, and let users decide if and how they want to handle input. This would be the simplest option, but would also imply that they need to handle the maths ([1] [2] [3]) for the selected colour.
2. HueSaturationPicker, SaturationValuePicker, etc. types, which are controls that handle input and the selected colour for the user.