shp: document shape pos and size feature analysis
acpt: add shp-pos-and-size.feature
shp: add Shape.left and .top getters
shp: add Shape.width and .height getters
shp: add Shape.left and .top setters

along the way:

* extract Shape._ext and ._off
shp: add Shape.width and .height setters
shp: elaborate analysis for pic pos and size
acpt: add scenarios for picture pos and size
pic: add Picture.left and .top getters

along the way:

* refactor test_picture.py to pytest
* trim out dead code from oxml.unitdata.autoshape
test: rename unitdata.autoshape to .shape
pic: add Picture.width and .height getters
pic: add Picture.left and .top setters
pic: add Picture.width and .height setters