add exit on sync or execute on sync

[muzzol]

Feature description

Exit the program or execute script when all folders or specific one is fully synced.

My proposal is to add a global flag like --exit-on-sync
If no folder id is specified, all folders should be on Up to Date state to trigger the parameter.

With this parameter a simple script like this should work:

#!/bin/bash
syncthing --no-browser --exit-on-sync --folder-id=abcde-12345
poweroff

Alternatively a similar effect could be achieved with --execute-on-sync=my-script.sh

In any case, the goal is to have a very simple way to know when a folder is synced and give control back to the console and don't wait indefinitely.

Problem or use case

Users with different computers (home, studio, work) with a single folder but huge size, so that have to wait for hours or leave the computer on if they don't have physical access to it.

Alternatives or workarounds

There's a lot of different aproaches on the forums and issues but none is reliable because they use parsing files or programs.