New .page_mode & .viewer_preferences properties

Added

• new FPDF.page_mode property, allowing to display a PDF in full screen: link to docs
• new FPDF.viewer_preferences property: link to docs

Fixed

• removed a debug print() statement (multi_cell: new_x=... new_y=...) that had been left in multi_cell() method 🤦‍♂️
• preserved backward compatibility with PyFPDF for passing positional arguments to cell() & multi_cell(), which was broken in 2.5.2

Modified

• when regular_polygon() is called with style="f",
  the shape outline is not drawn anymore. Use style="DF" to also draw a line around its perimeter.

Deprecated

• the fill parameter of the polygon()
  & polyline() methods have been replaced by a style argument,
  offering more control