Support for text styling / rich text

[Toqozz]

There is currently no way to style individual parts of text within a text block -- you can only change the style of the entire block.

I've seen a couple different ways of doing this on the user end:

• Markup in strings, like in Pango/GTK+ (https://developer.gnome.org/pygtk/stable/pango-markup-language.html)
• Setting state when inserting text into the field, like in Java/Swing (http://www.java2s.com/Code/Java/Swing-JFC/StyledText.htm)

From a user perspective, I personally prefer markup strings in the text.

glyph_brush already supports text styling using glyph_brush_layout, so implementation should just be a matter of parsing the text in whatever format to a Vec<SectionText>.

[hecrj]

I think we should tackle this together with #34, once we figure out font fallback and text shaping (see #33).

String markup will force us to implement some kind of parser, while setting state is quite a side-effect heavy approach.

I personally like the approach of elm-ui. It's explicit, self-contained, and composable.

[Toqozz]

I like the elm-ui example, but I worry that it won't be as ergonomic in rust. The glyph_brush api uses a similar approach:

let glyphs = Layout::default().calculate_glyphs(
    &fonts,
    &SectionGeometry {
        screen_position: (150.0, 50.0),
        ..SectionGeometry::default()
    },
    &[
        SectionText {
            text: "hello ",
            scale: Scale::uniform(20.0),
            ..SectionText::default()
        },
        SectionText {
            text: "glyph_brush_layout",
            scale: Scale::uniform(25.0),
            font_id: FontId(1),
            color: [0.0, 1.0, 0.0, 1.0],
        },
    ],
);

[hecrj]

We currently use the builder pattern to configure widgets. I'd use the same strategy here.

However, we are not quite close to implementing this yet. The API could change considerably before we get to tackle this.