Optional
alphaRepresents the alpha channel of the color as u8 in the range 0..255.
Represents the blue channel of the color as u8 in the range 0..255.
Represents the green channel of the color as u8 in the range 0..255.
Represents the red channel of the color as u8 in the range 0..255.
RgbaColor represents a color in the Slint run-time, represented using 8-bit channels for red, green, blue and the alpha (opacity).