(format-subtract-regions): New function.

(format-subtract-regions): New function.
(format-property-increment-region): New function.

(format-deannotate-region): When multiple annotations
go into a single text property, split the outer annotations (with
format-subtract-regions) instead of resetting them; use lists of
regions instead of a single number for the text property start.

(format-deannotate-region): Don't change extents of
enclosing annotations of the same kind.

(format-deannotate-region): Use
property-increment-region to add to numeric properties.