Translations: make it easier to translate controller scripts

Currently it's necessary to override existing translations (https://doc.qt.io/qtinstallerframework/ifw-customizing-installers.html#configuring-and-overwriting-default-translations) in order to translate strings in a controller script.

Would be easier for the users if the translations could instead be added like they are added for packages (https://doc.qt.io/qtinstallerframework/qtinstallerframework-translations-example.html).