Resolve "Add amstrad gx4000 system"

Please make sure your MR is ready to be merged !

• You added the changes in CHANGELOG.md
• You described the MR as below
• You updated the recalbox wiki
• You wrote a blog post on recalbox-blog

Changes :

• This adds the amstrad gx4000 system with libretro’s cap32 core.
  There are still some games not working but that is being worked on atm: https://github.com/libretro/libretro-cap32/issues/59
• Tested ad working. Theme needs to be merged and package bumped.

Related to :

• recalbox-configgen!225 (merged)

Closes #669 (closed)