Kuwahara filter: Add to Effects >Blur menu

Also in this commit, I've removed Chroma Blur and Grid Blur from the
Blur menu.  Both of these require specialized implementations instead of
the approximations I'm currently using, and rather than revisit them
immediately, I'm leaving their respective code modules but simply
removing them from the menu.  (I probably won't revisit them until
post-6.4, FYI.)