docs: document cell fill analysis
acpt: add set cell fill scenario
tbl: refactor test for _Cell.text
tbl: refactor test for _Cell.margin_x getters
tbl: refactor test for _Cell.margin_x setters

along the way:

* rename CT_TableCell.__get_marX() to ._get_marX()
* factor out objectify calls as encountered
* don't remove empty <a:tcPr>
tbl: refactor test for _Cell.margin_x assign raises
tbl: refactor test for _Cell.vertical_anchor getter

along the way:

* add enum MSO_VERTICAL_ANCHOR
* delete pptx.spec.VerticalAnchor
* migrate enum idx to ST_ value translation to oxml
* change _Cell._get and _set_vertical_anchor to property decorator
*
tbl: refactor test for _Cell.vertical_anchor setter

along the way:

* standardize naming of fixtures like {property}_(get|set)_fixture,
e.g. margin_get_fixture
tbl: add _Cell.fill

along the way:

* add CT_TableCellProperties custom element class
* Describe_Cell.resequence it_can_set_the_text_it_contains()
* extract cell() test fixture
tbl: complete CT_TableCellProperties

Adding to xPr_bldr fixture in tests.dml.test_fill subjects it to full
fill test suite. Most of the added code should come out when xmlchemy
goes in.