introduce opcodes header

introduce opcodes header

This brings the opcodes in the C world. This requires a bit of macro
gymnastic to keep definitions in a central place. Opcodes are defined
like this:

  #define OPS(_fmt) \
    _fmt(OP_NULL, N_("null operation") \
    ...

By passing a different formator to `OPS`, one can keep only parts of the
definition she/he is interested in. See the end of opcodes.h for example
usages.

By introducing this new header, it is no longer necessary to generate
keymap_alldefs.h, which was used only for consumption by gettext.