[deansasek]

Last of the clipboard save stack, on top of #497, #498, #499 and #500.

Closes #470

Stacked on:

• Failed clipboard.json and history writes return silently, so a pin can look saved when nothing reached disk. #497 Failed clipboard.json and history writes return silently
• A copy still being built when Clear History runs is added afterward, so the cleared clip reappears. #498 A copy still being built when Clear History runs is added afterward
• Quitting can drop the last pin change, because the quit flush does not wait for the pinned-file write queue. #499 Quitting can drop the last pin change
• Copying a pinned clip again updates its date and source app in memory but never rewrites clipboard.json. #500 Copying a pinned clip again never rewrites clipboard.json
• Pins made after the Keychain key fails at launch are never written, because the failure is never retried. #470 Pins made after the Keychain key fails at launch are never written (this pull request)