doc: base _Body on Parented

Improved some names along the way, document_part instead of document
where an instance of DocumentPart.
tbl: base Table on Parented
tbl: base _Columns on Parented
tbl: base _Rows on Parented
tbl: base _Column on Parented
tbl: base _Row on Parented
tbl: base _ColumnCells on Parented
tbl: base _RowCells on Parented
tbl: base _Cell on Parented
txt: base Paragraph on Parented
txt: base Run on Parented

Remove stray print() call left in during testing.
shp: refactor InlineShapes.add_picture()

Key change is adding *run* parameter to InlineShapes.add_picture()
rather than adding a run to the document in the method, so picture can
be added to an arbitrary run.
acpt: clean up picture-related test steps

* rename helpers.test_file_path to test_file
* move picture-related assertions to steps/shape.py
* refactor given_a_run()
acpt: add run-add-picture.feature
txt: add Run.add_picture()
doc: refactor Document.add_picture()

Use Run.add_picture() to do the heavy lifting.