Put various preps into place, that enable us to read txbxContent from
group shape children via the writerfilter parser, which gets us much
more features

Also:
- store shapecontexthandler on wrapper class in writerfilter
- move adding children shapes to ctor, not dtor
- remove RuntimeException in Writer's XShapes::add(), aligns this
  with Draw/Impress API semantics, and helps here when trying to
  add the same shape a 2nd time (which we then simply ignore)
- make oox's Shape notion of 'inside group' less ad-hoc
- make SwTextBoxHelper::getTextRectangle() cope with more than
  CustomShape - so passing in a group shape will also yield some
  sensible bounding box
- have SwTextBoxHelper handle group content, too
- derive WpsContext from ShapeContext, so we can later substitute it
  for that inside WpgContext
- keep WpgContext::onCreateContext() _for the moment_ with the old
  delegation to ShapeContext (needs to use WpsContext to enable
  writerfilter text input)

Change-Id: I2b0f0583e21137f3321a8dc13823058b14d19773
Reviewed-on: https://gerrit.libreoffice.org/65914
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>