Make *areaEnabled(int area) a wrapper for *(QString categoryName).

This will let us replace uses of the former in favor of the latter and
eventually get rid of the old area enums and the map between enums
and categories.
Add {set,is}AreaEnabled wrappers taking QLoggingCategory.

This lets callers do stuff like setAreaEnabled(ORE_AUDIO(), true).
Port settings dialog to use category names to enable/disable logging.