Replace a special Color32::PLACEHOLDER with widget fallback color

[emilk]

This introduces a special Color32::PLACEHOLDER which, during text painting, will be replaced with TextShape::fallback_color.

The fallback color is mandatory to set in all text painting. Usually this comes from the current visual style.

This lets users color only parts of a WidgetText (using e.g. a LayoutJob or a Galley), where the uncolored parts (using Color32::PLACEHOLDER) will be replaced by a default widget color (e.g. blue for a hyperlink).

For instance, you can color the ⚠️-emoji red in a piece of text red and leave the rest of the text uncolored. The color of the rest of the text will then depend on wether or not you put that text in a label, a button, or a hyperlink.

Overall this simplifies a lot of complexity in the code but comes with a few breaking changes:

• TextShape::new, Shape::galley, and Painter::galley now take a fallback color by argument
• Shape::galley_with_color has been deprecated (use Shape::galley instead)
• Painter::galley_with_color has been deprecated (use Painter::galley instead)
• WidgetTextGalley is gone (use Arc<Galley> instead)
• WidgetTextJob is gone (use LayoutJob instead)
• RichText::into_text_job has been replaced with RichText::into_layout_job
• WidgetText::into_text_job has been replaced with WidgetText::into_layout_job

[Mingun]

Misprint: Renamed. Strange, why wasn't catched by CI