Add revert to backup option to file menu

[taylorobyen]

This addresses #3427

Load Backup button to File menu on the main window

When loading a backup the relative time it was taken is displayed

[dae]

Sorry Taylor, I appear to have made a thinko. When I suggested mentioning the previous backup time, it was because I was assuming we'd be automatically selecting the most-recent backup. Giving the user a list of backups is more flexible, and it makes the time shown somewhat redundant. I was originally going to ask you to move the "x minutes ago" code into Rust so it can be shared across the clients, but since it's a bit redundant, the fastest way to get the PR over the line would be to change the message to show the backup filename instead of a relative time. Does that work for you?

[taylorobyen]

No worries Damien, I've switched it to be filename as requested. 👍

[dae]

(will put this through myself; was a pending suggestion that I forgot to send)

[dae]

Thanks Taylor!