nls: change <Compose> <^> <-> to mean superscript minus instead of macron

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.

Requested-by: J. McWilliams

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>