Support DesignSpace v5.x data

[madig]

• Axes elided fallback name https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#axes-element
• Discrete axes https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#axis-element
• Axis labels
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#labels-element-axis
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#label-element-axis
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#labelname-element-axis-stat-label
• Mappings element
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#mappings-element
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#mapping-element
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#input-element
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#output-element
• Top-level labels element
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#labels-element-top-level
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#label-element-top-level
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#location-element-top-level-stat-label
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#dimension-element
  • https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#labelname-element-top-level-stat-label
• Localised names for sources https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#familyname-element-localised-names-for-sources
• Variable fonts element and subelements https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#variable-fonts-element
• Instance element
  • Optional location attribute https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#instance-element
  • fontinfo dicts https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#example-for-varlib