Rich text for all widgets

[emilk]

This introduces the new types RichText and WidgetText, used for specifying text colors etc for Button, Checkbox, RadioButton, CollapsingHeader and Window.

This enables user to have fine control over the labels used in many widgets, for instance to change their fonts, colors, add underlining, or even mix fonts and colors.

WidgetText can be created from a simple String, from a RichText, LayoutJob or a Galley. The last two are the most powerful, but least ergonomic.

RichText is a builder that takes a String and can then apply color, TextStyle, strikethrough, underlining, etc.

Splitting out text formatting from the widgets makes them more orthogonal, and less code is needed in the widget implementations.

Code breakage

Most old code should work with a helpful deprecation notice.

// Before:
ui.add(Button::new("delete").text_color(Color32::RED))

// After:
ui.add(Button::new(RichText::new("delete").color(Color32::RED)))
// or:
ui.button(RichText::new("delete").color(Color32::RED))


// This also works now:
ui.checkbox(&mut my_bool, RichText::new("delete").color(Color32::RED));

TODO:

• Use WidgetText in Button, Checkbox, RadioButton, CollapsingHeader, SelectableLabel, Window title
• Add more builders to RichText: italics, code, underline strikethrough, weak, …
• Use WidgetText in Label
• Use WidgetText in Hyperlink
• Use WidgetText in ComboBox
• Do not re-layout WidgetText::Galley.
• Benchmark (faster by around 5%, thanks to avoiding a text clone in Label)
• Add a line to CHANGELOG.md
• More places in ComboBox
• Menu
• Painter too complicated
• Any place with #[allow(clippy::needless_pass_by_value)]
• Any place with impl ToString
• Check if there are any more places that could make use of RichText.
• Plots later PR