Text strikethrough and underline colors

[ickshonpe]

Objective

Allows users to override TextColor and use a custom color for underline and strikethrough text decorations.

Solution

New components UnderlineColor and StrikethroughColor. Add them to a text entity with Underline or Strikethrough, respectively, to use a custom color for the lines.

This could just be an optional color field on Underline or Strikethrough, that would be fine. I spent a while going back and fourth about it. The main reasons why I went with a separate component:

• The same color is almost always used for both text and its decorations. Overriding TextColor is going to be rare.
• Once we have some sort of global or propagated text style support, underline and strikethrough colors probably won't be set per entity.

The single component approach has some advantages too. Discoverability, obviously. Also compare:

(
    Text::new("Hello"),
    Underline,
    UnderlineColor(RED.into())
)

with

(
    Text::new("Hello"),
    Underline::color(RED),
)

which seems clearly nicer. If it weren't for the planned non-entity-local text styling support, I'd lean more towards the single component approach.

Testing

Added a few color overrides to this example:

cargo run --example strikethrough

Showcase

[mnmaita]

Looks good, and using separate components for the colors sounds sensible to me.