Implement support for importing/exporting images

What's Changed

• Implement single frame image saving by @haroldo-ok in #87
• Implement button for importing a image by @haroldo-ok in #88
• Improve exported filename generation by @haroldo-ok in #89
• Bump version to 0.21.0 by @haroldo-ok in #90

Full Changelog: v0.20.0...v0.21.0