Automatically merge translation files at build time

Support automatic merging of .qm files at build time.

Often translations of an application is split up in multiple .ts files . For instance the Qt libraries themselves do support translations for their user visible strings in a modular fashion.

The traditional way to load these .qm files is by loading and registering multiple QTranslator objects, at runtime. Anyhow, this has some challenges:

• You typically do want to load a consistent set of translations. Custom code is needed to handle situations where , for some language, only some .qm files exist.
• Conflicting translations between .ts files are not detected.
• macOS expects a list of supported languages as part of the app bundle metadata. (This is solved as of Qt 6.7. One can specify I18N_TRANSLATED_LANGUAGES in qt_standard_project_setup, and that information is written to the bundle meta data.)

It would be therefore better to semi-automatically merge translations at build time, for instance with the lconvert tool. Such a synthesized .qm file could then also be named so that the logic in QQmlApplication will automatically load it.