Sync only at the end of file transfers to fix performance regression

[vjr]

Fixes the slowdown introduced by #2828 to fix #2818 while preserving the intent of keeping users informed that their file transfers are in progress so they don't inadvertently unplug their external/removable storage.

The progress dialog now shows "finishing copy" or "finishing move" briefly at the end while the transfer is being synced.

Tested copy/move between fast/slow internal/external storage - fast nvme ssds and slow usb sticks.

[jeremypw]

@vjr Performance aside, I think main feels more natural with slow devices. With this PR, moving one large file onto a slow drive immediately shows progress of 100% (transfer to cache) but the progress window stays open for a long time showing "Finishing ..." but giving no indication of how long it will take. In main you get a better idea of the true rate of progress. For slow drives syncing often makes little performance difference, whereas for SSDs it makes a more significant difference. Not sure how to solve this in a simple manner - its probably not worth spending too much time on tbh. I'll maybe revisit after porting copy/move to Vala.

[jeremypw]

@vjr I notice your first solution was to limit syncing on a time basis which seems reasonable (together with syncing after transferring the last file). What was the reason you tried a different solution?