Persist metadata in GIF file for iCloud

[sindresorhus]

We add metadata that the GIF was made with Gifski, but that metadata is currently not preserved when the GIF file is synced with iCloud.

Gifski/Gifski/ConversionViewController.swift

Line 87 in 8e152d3

try? gifUrl.setMetadata(key: .itemCreator, value: "\(App.name) \(App.version)")

See: https://eclecticlight.co/2019/07/23/how-to-save-file-metadata-in-icloud-and-new-info-on-extended-attributes/

[sindresorhus]

I tried implementing this, but it turns out, if you set the sync flag, it no longer shows up in the Finder preview pane. I would rather have it show in Finder than being syncable.

[sindresorhus]

The bounty is moved to #219