A mechanism to update a PrintBox logs display in real time

[lukstafi]

Get the benefits of the Flushing backend with the PrintBox backend. This is a tough ask, in the meantime we fortunately have Flushing.

[lukstafi]

Add a snapshot function to output all pending material, by virtually (temporarily) closing entries, but preserving the logging state (so calling snapshot has no effect on the eventual full logs). Add a configurable auto-snapshotting option, that at each logging event, snapshots logs if sufficient time has passed. For file based outputs, truncate the file to remove old snapshots when generating new ones or full logs. For text based outputs (especially if defaulted to the stdout channel), optionally add "clear terminal" symbols to mimic file truncation.