0.18

• Add extension that allows use of custom framerates (#637503)
• Add "Get more extensions" button to the preferences dialog
• Relax SubRip file parsing in unambiguous cases (#634129)
• Fix saving of last used directory in file dialogs with "paths that
  cannot be represented as a local filename" (#649347)
• Add Brazilian Portuguese translation (Átila Camurça,
  Darlildo Souza)
• Update German translation (Chris Leick)