oox: add support for the wpg namespace

The plan is that once these are here, the writerfilter ooxml tokenizer
can refer to them. And then the writerfilter will call back to oox to do
the real drawingml import.

Change-Id: I00eec562e32df359231d1a170367ab75702f0049
writerfilter: tokenize wpg:wgp

These are just the minimal changes, so that writerfilter calls into
oox::shape::ShapeContextHandler, which does the real work.

Change-Id: I7830178efd6ed75da6737a67d9d24dcf59337954
oox: initial import of wpg:wgp

We import something that's visible, but the position of the shape is not
correct yet.

Change-Id: Ie68f0ebad1cc992a6c8d7704d7262f7e983f3b19