Fixes portuguese translation

Typos and inconsistencies:
- "Contracto" changed to "Contacto" (contract vs contact)
- "mensagem" (message) was mistyped a few times
- "apagar" and "deletar" were both used. In pt-PT, "deletar" is not a thing,
  and there's a separate pt-BR translation, so I decided to go with "apagar".

- "plaintext" was translated both as "texto simples" and "não cifrado/a". The former
  is a literal translation, and the latter conveys the fact that the text is not encrypted.
  Decided to go with "não cifrado" ("not encrypted").

Subjective change:
- "intercâmbio de chaves" changed to "troca de chaves" (key exchange).
  It's a simpler word, with the same meaning.