refactor: rename test/ to tests/
reorg: migrate text-related steps to own module
reorg: migrate table-related steps to own module
reorg: migrate placeholder-related steps to own module
reorg: migrate core props steps to own module
reorg: migrate picture-related steps to own module
reorg: migrate shape-related steps to own module
reorg: migrate slide-related steps to own module
reorg: rename prs_steps.py to step_presentation.py
reorg: migrate step working constants to helpers.py
reorg: migrate text classes to pptx/text.py
reorg: migrate table classes to pptx/table.py

Also needed to move _BaseShape to new pptx/shape.py to solve a circular
import problem, will see how that settles out.
reorg: migrate autoshape classes to autoshape.py
reorg: migrate placeholder class to placeholder.py
reorg: migrate _Picture to picture.py
reorg: rename shapes.py to shapetree.py
reorg: move shape modules to pptx/shapes/
reorg: migrate shape tests to tests/shapes/
reorg: partial breakup of presentation.py
rels: remove _BasePart dependency in notify