New vector drawing API & SVG embedding

Added

Thanks to @torque for contributing this massive new feature:

• add fpdf.drawing API for composing paths from an arbitrary sequence of lines and curves.
• add fpdf.svg.convert_svg_to_drawing function to support converting basic scalable vector graphics (SVG) images to PDF paths.

Fixed

• will_page_break() & accept_page_break are not invoked anymore during a call to multi_cell(split_only=True)
• Unicode characters in headings are now properly displayed in the table of content, cf. #320 - thanks @lcomrade