Originally written by @Crzyrndm, he is not active and gave permission for me to continue this. Original thread is here: https://forum.kerbalspaceprogram.com/index.php?/topic/93955-130-filter-extensions-304-jul-11/
Release is here:
With the stock editor now supporting custom filters in a reasonable way, creating more and better filters is the next logical step. However, the requirement for categories to be managed via code is just a terrible idea. Not to mention that there is no easy