This is quite hacky and limited: in OOXML these are not OLE objects but
occur inside text boxes, and PPT 2010 allows inserting multiple Math
objects into one text box but Impress does not have as-character
anchored objects, so we can't import that properly; for now only import
Math if there is nothing else in the text box.

Also for now only import them as children of TextParagraphContext (a:p);
it's not clear what the possible parent elements could be since the
OOXML standard only lists WordProcessingML parent elements :(

Change-Id: I847f810084c9ddae4b60f93896fb73a742683cc2