[Meta] RTL / BiDi support

• #1025 (closed): bugs in the experimental mml3.js extension
• UI issues
  • #628 (closed): menu location)
  • #601 (closed): mixing RTL and LTR in the UI
• #474: Complex Layout (e.g., Unicode characters)
• #610: RTL support in MathML
  • hacks for TeX input: https://github.com/mathjax/MathJax/issues/610#issuecomment-135366695
  • #1308: support mirrored roots