Launcher discards launcher.cfg settings it does not recognise

Since !2650 (merged) the launcher only keeps the launcher.cfg fields it knows about. Anything else is logged as an unsupported setting and dropped on the next save. Before that, every line was kept and written back, so an older launcher left alone whatever a newer one had added.

That means any new field added to launcher.cfg is erased by running an older launcher, which affects anyone switching between releases or RCs to test something. First seen with the order= field in !5592 (merged), where the 0.51 launcher warns about it and drops it.

Label requested: Long-term Regression (plus Bug, Launcher). This account cannot set labels on the project.