Polygons, accessibility & structure tree

Added

• FPDF.polyline & FPDF.polygon : new methods added by @uovodikiwi - thanks!
• FPDF.set_margin : new method to set the document right, left, top & bottom margins to the same value at once
• FPDF.image now accepts new optional title & alt_text parameters defining the image title
  and alternative text describing it, for accessibility purposes
• FPDF.link now honor its alt_text optional parameter and this alternative text describing links
  is now properly included in the resulting PDF document
• the document language can be set using FPDF.set_lang

Fixed

• FPDF.unbreakable so that no extra page jump is performed when FPDF.multi_cell is called inside this context

Deprecated

• fpdf.FPDF_CACHE_MODE & fpdf.FPDF_CACHE_DIR in favor of a configurable new font_cache_dir optional argument of the fpdf.FPDF constructor