Add new color variants to iced_core::Color.

[edfloreshz]

I think it would find it useful to have more colors available in the Color struct and not have the user construct their own, I was thinking of adding more constants to it.

Another approach would be to create a Colors enum with different variants of colors, something like this:

/// A set of default colors.
#[derive(Debug)]
pub enum Colors {
    /// The black color.
    Black,
    /// The white color.
    White,
    /// A color with no opacity.
    Transparent,
    /// The red color.
    Red,
    /// The blue color.
    Blue,
    /// The green color.
    Green,
    /// The yellow color.
    Yellow,
    /// The orange color.
    Orange,
    /// The pink color.
    Pink,
    /// The purple color.
    Purple
}

Let me know your thoughts.