Unify Qt Quick Controls style-specific attached properties API into a general styling API

Currently, different styles offer several different attached APIs to customize the controls.

For example, the Material style offers API to customize: the elevation, the radius of the rounded corners of the controls, container style (eg: filled, outlined), drop shadow, etc.

These are APIs that can be shared among the different styles. For example, the iOS style can make use of the container style property to customize the background of the containers for several controls: eg: Button, TextField, etc. , since this is something customizable for the UIKit controls. The same can be said for the rounded corners property.

Also, having configurable properties for drop shadow, elevation, rounded corners, border style, etc would be useful for all styles as additional ways to customize them.

On top of that, almost all styles have their own different attached APIs for customizing the colors of different components, many of which are duplicated among the different styles. For example, both the Material and Universal style have properties for: backgroundColor, foregroundColor, accentColor, etc. Many of these properties can be replaced by the existing palette API. Some others, might require extending the Palette and adding more color roles.

These are the affected properties for Material style:

• Material.background
• Material.foreground
• Material.elevation
• Material.roundedScale
• more?