clock: Rename timestamp to file_timestamp && time(void) to timestamp

* This removes the undesirable method overload (of nom::time) and
offers a more descriptive naming scheme for the difference
between nom::timestamp && nom::file_timestamp.

* Remove const keyword from return types; returning by value does
not apply.
tests/CMakeLists.txt: Introduce  NOM_TESTS_VTEST_SOURCE var

* This helps separate the dependencies...
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