Changes of version 15.4.0

• Add characters:
  • LATIN SMALL LETTER SIDEWAYS O (U+1D11);
  • LATIN SMALL LETTER SIDEWAYS OPEN O (U+1D12);
  • LATIN SMALL LETTER SIDEWAYS O WITH STROKE (U+1D13);
  • LATIN SMALL LETTER SIDEWAYS U (U+1D1D);
  • LATIN SMALL LETTER SIDEWAYS DIAERESIZED U (U+1D1E);
  • LATIN SMALL LETTER SIDEWAYS TURNED M (U+1D1F);
  • MODIFIER LETTER SMALL SIDEWAYS U (U+1D59);
• Make user build plans' variants inheritable (#1345).
• Add top-cut variants of 1 (#1354).