Several other <Multi_key> <asciicircum> <symbol> sequences
produce the superscript equivalent of the given symbol. So,
let <Multi_key> <asciicircum> <minus> do the same.
Also, add two other sequences for producing a plain macron, to compensate a bit the loss of the above sequence.
Additionally, make <Multi_key> <underscore> <minus> produce
a subscript minus, for consistency.
This fixes issue #165 (closed).
Requested-by: J. McWilliams
Signed-off-by: Benno Schulenberg bensberg@telfort.nl