Update code to latest API in nomlib's feature/GUI dev branch

* Introduce customized logging category levels; this significantly
helps separate the run-time logging output between nomlib & the
game.

* Remove NOM_DEBUG_TRACE & co variables; these have been phased out by
moving to run-time control of logging (via SDL2 logging facilities).

* Use the appropriate logging categories for all NOM_LOG_TRACE calls
made in the project.

* Initialize / optimize logging priorities to only show game state
entry/exit traces.

* Replace nom::Cursor inheritance with nom::AnimatedSprite; this
entailed phasing out the x & y method helpers, along with a
rename of the ::position method to ::cursor_position so that
we could take advantage of the existing ::position method available to
nom::AnimatedSprite.

* Update GUI subsystem code to the latest feature/GUI dev branch
API of nomlib; everything is functioning 100% as of this point and
time, AFAIK, less & except the noted FIXME regarding absolute &
relative window coordinates of the parent (top-level) GUI window
objects (for nom::MessageBox objects).