Add 12, 8, and 6 FPS presets often used in stylized animation,
especially in 2D/Grease Pencil animation.
12 FPS to animate on 2's, 8 FPS on 3's, 6 FPS on 4's or for
early blocking stages or storyboard.
Pull Request: https://projects.blender.org/blender/blender/pulls/135712
