Add support for adding text and image watermarks to documents.

API Design

• section.add_text_watermark(text, font='Calibri', size=Pt(72), color=RGBColor(0xC0,0xC0,0xC0), layout='diagonal')
• section.add_image_watermark(image_path, width, height)
• section.remove_watermark()

Implementation

Watermarks are implemented as shapes in the header:

• Text watermark: VML shape (v:shape with v:textpath) in the default header
• Image watermark: VML shape with v:imagedata in the default header
• Modern Word also supports DrawingML-based watermarks

Needs to add/modify the header part for each section.

Upstream: python-openxml#845 (8 comments)