Add a list of available baseline tags to GimpTextBuffer and
automatically create baseline tags as needed. They get serialized as
<span rise="value">, so add attribute and value parameters to the
buffer's tag_to_name() and name_to_tag() functions. Properly managing
the rise's amount is a TODO, currently each keystroke changes the
baseline by 1024 pango units, which might be whatever depending on the
output grid.
When turning overlapping tags into a strictly nested markup structure,
make sure we don't treat tags that are ending at an iter like tags
that were only closed because overlapping spans don't exist in markup.
Chew on this sentence a bit, it took ages to write it.
- 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.
