Make RichText generic over data structure

... and decouple `markdown::parse` from theming

core/src/padding.rs

@@ -32,7 +32,7 @@ use crate::{Pixels, Size};
 /// let widget = Widget::new().padding(20);              // 20px on all sides
 /// let widget = Widget::new().padding([10, 20]);        // top/bottom, left/right
 /// ```
-#[derive(Debug, Copy, Clone, Default)]
+#[derive(Debug, Copy, Clone, PartialEq, Default)]
 pub struct Padding {
     /// Top padding
     pub top: f32,

core/src/text.rs

@@ -252,7 +252,7 @@ pub struct Span<'a, Link = (), Font = crate::Font> {
 }
 
 /// A text highlight.
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Highlight {
     /// The [`Background`] of the highlight.
     pub background: Background,