test: refactor DescribeTextFrame
test: refactor Describe_Run
docs: document text access and mutation analysis
acpt: add scenarios for text access and mutation
test: refactor Describe_Paragraph
txt: add Paragraph.text getter, setter takes '\n'

* add a:br and a:fld elements as contributors to text content in a
paragraph
* add _ParagraphTextAppender service class to take care of parsing text
for line feeds and getting <a:br> elements in the right place
txt: add TextFrame.text getter
txt: Add default values for TextFrame.margin_*

* Previously returned None where no xIns attribute was present, but
  better to return the well known defaults, even though it obscures from
  the API whether an xIns attribute is actually present.

* remove tests/oxml/test_text.py; it no longer tests original
  functionality now that xmlchemy takes care of the low-level details
  in the oxml layer
rfctr: migrate BaseShape.text setter to Shape
acpt: add scenarios for Shape.text
shp: add Shape.text getter

* elaborate _Paragraph.clear() to also remove line break and field
elements.
* fix ordering of text property in class
rfctr: move BaseShape.textframe to Shape

* Access strategy changes now that txBody is guaranteed to be a valid
  child element of the shape. Use get_or_add_txBody() and eliminate
  raise of ValueError.
* Requires txBody on p:sp and non-default _new_txBody to add required
  child elements.
rfctr: split has_textframe btwn BaseShape & Shape
rfctr: rename textframe to text_frame

* Deprecate Shape.textframe and _Cell.textframe
* Remove old design/protcols pages