xmloff: fix import of tables inside textboxes inside shapes

We already had the concept of having a XML_TEXT_TYPE_SHAPE type (for
editeng-based text) and a XML_TEXT_TYPE_TEXTBOX (for Writer-based text),
but in case of draw:custom-shape, the previous was always assumed,
implicitly.

Check if the shape is marked as having a textbox, and if so, use the
XML_TEXT_TYPE_TEXTBOX handler, which makes e.g. the table import work.

Change-Id: I45f28b36df1836eeba89e9eef44c7abd87fcbd56