because it breaks the invariant that the pango layout used for
navigating in the on-canvas editor must be a 1:1 view on the text
buffer used for editing. Will have to fix that somehow...
- keep around tags for styles in GimpTextBuffer. For now only bold,
italic, underline and strikethrough.
- add GimpTextStyleEditor, a widget which allows setting tags on
a GimpTextBuffer's selection.
- add serialize/deserialize code to/from pango markup using
GtkTextBuffer's rich text (de)serialization infrastructure.
Doesn't produce or handle <span> yet.
