Support initializing language from argument or query parameter

[chriadam]

No description provided.

[blammit]

Would be nice if the language could be set from the BackendConnection or such before the rest of the UI was loaded (e.g. like how gui-v1 controls the temperature value in https://github.com/victronenergy/gui/blob/master/src/user.cpp) so that SystemSettings.qml wouldn't have to manage the startup case. But if that doesn't work out, then LGTM.

[chriadam]

Need to reconsider this, as we don't really want the query parameter to overwrite the system setting. Instead, it should be more like: "what language should the error messages at startup time be, e.g. if it cannot connect to the MQTT broker?"
Not sure. If that's the case, merely setting the language in the singleton should be enough (and then it would automatically be overwritten once connection to MQTT succeeds, based on the system language setting received via MQTT).

[MikeTrahearn-Qinetic]

This section seems out of date with main at the time of writing this comment, but it is noticed that the Victron::VenusOS::Language::create(&engine); does not specify QQmlEngine::setObjectOwnership(language, QQmlEngine::CppOwnership); and could cause multiple instances if the create() function was called multiple times. Suggest using Meyer's singleton pattern to prevent this.