app: support baseline changing via Alt+Up and Alt+Down
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.
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