cad/solvespace: Update to 3.0 and take maintainership

• Update to 3.0:
  • Update libdxfrw to 0b7b7b7
  • Introduce microsoft/mimalloc at 38a0322
  • Use gtmm 3.0
• pet portlint
• WWW: use https

Changes:

• https://github.com/solvespace/solvespace/blob/master/CHANGELOG.md#30