message: gui: Update transifex slug for 0.19
diff: [main]
  host = https://www.transifex.com
  -[bitcoin.qt-translation-018x]
  +[bitcoin.qt-translation-019x]
  file_filter = src/qt/locale/bitcoin_<lang>.ts
  source_file = src/qt/locale/bitcoin_en.ts
  source_lang = en
  