This property was not implemented when the annotation body components
were converted from Angular to Preact.

 - Add `textStyle` prop to `MarkdownView` and `MarkdownEditor` which
   sets additional CSS properties to apply to the rendered annotation
   body
 - Read the theme style properties for annotation bodies in
   `AnnotationBody` and pass the resulting `textStyle` down to
   `MarkdownEditor` and `MarkdownView`

Fixes #2444