DataViewListTest: Clean up logging output

* Use NOM_LOG_CATEGORY_TEST for variable dumps & disable the logging
priority level for it by default.
[SQUAHS ME] Implement action trigger bindings for mouse button clicks
Refactor SDL2Logger to use abstracted logging priority enum

* SDL2Logger: Introduce nom::LogPriority enumeration -- deriving from
SDL_LOG_PRIORITY for the time being) -- in future preparation for
abstraction of the logging subsystem. All SDL_LOG_PRIORITY enumerations
have been replaced throughout the engine.

* SDL2Logger: Introduce convenience static methods for conversion to
and fro SDL_LogPriority and nom::LogPriority.

* SDL2Logger: Introduce humna-friendly string descriptions of each
priority level; this will eventually be used for things like dynamic
runtime logging via command-line switches. Note its particularly nice
use in the callback example in SDL2LoggerTest as well.

* src/CMakeLists.txt: Relocate building SDL2Logger & its dependencies --
system/clock.cpp, system/ObjectTypeInfo.cpp, system/IObject.cpp -- to
NOM_BUILD_CORE_UNIT, the engine core; this better suits the overall
organization, and it allows us to skip building the system unit in many
more circumstances.