[WIP] `artists.stitch_to_animation`: better gifs

[ddkohler]

Changes

Rework wt.artists.stitch_to_animation: more settings to customize the quality and size of the generated gifs.

• kwarg ignore_alpha to remove transparency from gif palette.
• kwarg reduce to easily shrink gif image size relative to input
• replace ImageIO dependency with pillow, since that is the only plugin we use.
• resolves #1182

Checklist

• [ ] added tests, if applicable
• [ ] updated documentation, if applicable
• [ ] updated CHANGELOG.md
• [ ] tests pass

[ddkohler]

closing to rebase changes specific to this off of master