Some dialogs were not being translated and in this PR I add fixes for that.
The AboutDialog is an Adwaita system dialog, but it wasn't being localized on Windows or macOS (https://github.com/danirod/cartero/issues/56). This was not happening on GNU/Linux and on macOS if libadwaita is still installed via Homebrew. It turns out that if you have dependencies with locale files, you have to distribute them with your app, otherwise the strings will not be present.
While I'm on this, it seems that I forgot to add the shortcuts dialog to the POTFILES.in file, so the strings in that file were never proposed for localization. This PR also adds the file to the POTFILES and provides strings in Spanish.
Some dialogs were not being translated and in this PR I add fixes for that.