TS_FILE_BASE argument of qt_add_translations ignored by QQmlApplicationEngine

When using qt_add_translations, the default name format of .ts files is qml_<language-code>[_<country-code>].ts

The prefix of the name ("qml" by default) can be changed using TS_FILE_BASE. When it is set, the translation files are generated correctly.

However, QQmlApplicationEngine doesn't pick up on it and assumes the prefix is "qml" by hardcoding it in void QQmlApplicationEnginePrivate::_q_loadTranslations(). This can lead to translation files not being found at run time.