Add Link support to rich_text widget

[hecrj]

This PR introduces hyperlink support to the rich_text and span widgets.

The link type is generic; so you do not necessarily need to rely on strings for your links:

enum Message {
    LinkClicked(Link)
}

enum Link {
    Rust,
    Iced,
}

rich_text![
    "Here are 2 links: ",
    span("Rust").link(Link::Rust),
    " and ",
    span("Iced").link(Link::Iced)
]
.on_link(Message::LinkClicked)

The markdown widget does use strings as links, for obvious reasons.

[tarkah]

Looks great!