Support for subscript & superscript - set_page_background - embed_file - set_char_spacing

Added

• Added support for subscript, superscript, nominator and denominator char positioning as well as <sub> and <sup> HTML tags, thanks to @gmischler: link to documentation
• set_page_background(): new method added by @semaeostomea: link to documentation
• embed_file() & file_attachment_annotation(): new methods to add file attachments - link to documentation
• A new method set_char_spacing() allows to increase the spacing between individual characters, thanks to @gmischler: link to documentation
• workaround by @semaeostomea to support arabic and right-to-left scripts: link to documentation
• documentation on shapes styling: link to documentation
• documentation on sharing the images cache among FPDF instances: link to documentation

Changed

• HTML headings are now rendered with an additional leading of 20% the font size above and below them; #520
• fpdf2 now uses fontTools to read and embed fonts in the PDF, thanks to @gmischler and @RedShy

Fixed

• Text following a HTML heading can't overlap with that heading anymore, thanks to @gmischler
• arc() not longer renders artefacts at intersection point, thanks to @Jmillan-Dev; #488
• write_html():
  • <em> & <strong> HTML tags are now properly supported - they were ignored previously; #498
  • bgcolor is now properly supported in <table> tags; #512
• the CreationDate of PDFs & embedded files now includes the system timezone