Refactor logging subsys using SDL2 logging facilities

* Remove NOM_DEBUG_TRACE preprocessor definition; this feature is
now controlled at run-time (see NOM_LOG_CATEGORY_TRACE).

* platforms.hpp: Rename __func__ to NOM_FUNC.

* init.cpp: Enable all logging for the default NOM category. Disable
function call traces on NOM_LOG_CATEGORY_TRACE category levels, which
does not yet have any effect, since we are still using the legacy NOM
logging category (meaning traces will be enabled for everything).

* SDL2LoggerTest: Introduce unit test for SDL2Logger

* examples/macros.cpp: Clean up file a bit
Rename log source files SDL2Logger
UIBoxLayout: Fix compile err

* This err is due to the recent refactoring of nomlib's logging
subsystem; the macro signature has changed slightly.