meta-bug: table improvements

Meta-issue to collect improvements for mtable implementation

• displaystyle inheritance, #839 (closed)
• styling of rows #859 (closed)
• relative width in SVG #757 (closed)
• implement colspan, rowspan #1102
  • this would enable macros like border matrix, split #390
• implement elementary math, cf. #1101 (closed) (ok, not strictly mtable but obviously related)
• linebreaking within tables #957 (closed)