Scenarios list not loading on Linux

[jwrober]

Describe the bug
From the start screen, when you click the button to load a scenario game, the list of scenarios is empty.

To Reproduce
Steps to reproduce the behavior:

1. Start the client
2. Click on start scenario game
3. See error

Expected behavior
You should get a list of the scenarios installed.

Screenshots
This is the issue...

This is what I expect...

Platform and version (please complete the following information):

• OS: Linux (Debian and Arch)
• Freeciv21 version: 3.0-beta2
• Ruleset/Longturn game (if applicable): N/A

Additional context
During discord conversation it seems that in utility/shared.cpp the freeciv_storage_dir() is looking for a writable location, but the out of box scenarios are not installed in a writable location when installed via the package.

[lmoureaux]

During discord conversation it seems that in utility/shared.cpp the freeciv_storage_dir() is looking for a writable location, but the out of box scenarios are not installed in a writable location when installed via the package.

static QStringList default_scenario_path()
{
  return {QStringLiteral("."), QStringLiteral("data/scenarios"),
          freeciv_storage_dir()
              + QStringLiteral("/" DATASUBDIR "/scenarios"),
          freeciv_storage_dir() + QStringLiteral("/scenarios")};
}

QString freeciv_storage_dir()
{
  static QString storage_dir;
  if (storage_dir.isEmpty()) {
    // Make sure that all exe get the same directory.
    auto app_name = QCoreApplication::applicationName();
    QCoreApplication::setApplicationName("freeciv21");
    storage_dir =
        QStandardPaths::writableLocation(QStandardPaths::AppDataLocation);
    QCoreApplication::setApplicationName(app_name);

    qDebug() << _("Storage dir:") << storage_dir;
  }

  return storage_dir;
}

We should look in default_data_path instead, basically default_data_path() with /scenarios appended:

static QStringList default_data_path()
{
  // Make sure that all executables get the same directory.
  auto app_name = QCoreApplication::applicationName();
  QCoreApplication::setApplicationName("freeciv21");

  auto paths =
      QStringList{QStringLiteral("."), QStringLiteral("data"),
                  freeciv_storage_dir() + QStringLiteral("/" DATASUBDIR),
                  QStringLiteral(FREECIV_INSTALL_DATADIR)}
      + QStandardPaths::standardLocations(QStandardPaths::DataLocation);
  QCoreApplication::setApplicationName(app_name);
  return paths;
}