graphics/gegl3: Fix conflict with sysutils/coreutils

Both ports install bin/gcut. graphics/gegl3 already renames
bin/gegl -> bin/gegl3, so rename bin/gcut -> bin/gcut3 as well.

Set EXAMPLESDIR and let USES=uniquefiles do the renaming instead of
moving files manually.

While here also use options target helpers for EXAMPLES.

PR: 223462
Reported by: FreeBSD@ShaneWare.Biz
Approved by: gnome (maintainer timeout, 2 weeks)