L10N: add more helpful comments on OPS entries.
Now translators can see exactly where the entries came from.
Also, OPS.* can have L10N comments. Add "/* L10N: blabla */" to OPS files.
This commit would help translators a lot.
For example, the msgid "delete the current account" once surprised me. I said, "What? Can mutt delete my account???"
With this commit, I can see that "the account" is clearly the "autocrypt" account.
#. L10N: Help screen description for OP_AUTOCRYPT_DELETE_ACCT
#. #: ../OPS:8
#: ../keymap_alldefs.h:29
msgid "delete the current account"
TODO: make sure it is POSIX-compliant, i.e. it doesn't use GNU-extensions.