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.