Additional clean up of SDL2Logger feature

* Update (add & clean up) documentation / comments for SDL2Logger

* SDL2Logger: Consolidate the the one-time init of default priorities
logging levels from nom::init_third_party into SDL2Logger::initialize.

* config.hpp.in: Remove conditional preprocessor for logging interface;
it has been obsoleted by the fact that we can (and should) control
logging priorities at run-time.
* config.hpp.in: Mark NOM_DUMP macro deprecated.
* config.hpp.in: Add category argument to NOM_DUMP_VAR; dependent code
has been updated to reflect this change by logging to the appropriate
categories.

* RenderWindow: Use SDL2's err message when the window or renderer pointer
is invalid (instead of our own custom message).

* system/init.cpp: Use NOM_LOG_CATEGORY_TRACE_SYSTEM for nom::init and
friends.
* system/init.cpp: Uncomment logging of initialization messages; the
logging now occurs only when NOM_LOG_CATEGORY_SYSTEM is set to info
priority level or higher (default is CRITICAL).

* PlatformSettings: Uncomment logging of initialization message; the
logging now occurs only when NOM_LOG_CATEGORY_SYSTEM is set to info
priority level or higher (default is CRITICAL).

* SDL2LoggerTest: Introduce sanity test for using a custom log output
function
SDL2Logger: Fix corrupted output on WindowsOS

* SDL2Logger: Remove output_string and output_cstring methods