Add a stale file checker

[cj-wong]

Currently, there is no way to quickly observe if a file is stale. Since the photo frame indiscriminately loads images from USB storage and the timestamp on each image is in small font, I think a worthwhile enhancement is to include a file checker, most likely within the utils module.

I plan to have the image look something like...

Stale Files

Some files ($number) are potentially out of date:

- photo-dash-module: $hours_out_of_date

...and so on, maybe listing all modules (and overflowing if necessary onto more images).

Some things that should be considered:

• This functionality should be optional to preserve current behavior (so it will be enabled via the configuration file).
• Quiet hours may need to be accounted for $hours_out_of_date.