Gif manipulation sample

[tocsoft]

once SixLabors/ImageSharp#344 is merged and released to nuget create a sample manipulating frames for gifs.

Things to cover:

• Add Frame clones the source frame
• Export removes a frame
• Can't export last frame
• Show saving all frame from image

[tocsoft]

Make sure we cover SixLabors/ImageSharp#284 where the user want to save individual frames of an image.

[JimBobSquarePants]

Ooft. 5-year-old issue. I should, probably look into that!