[UX] Add loading indicator for USB device sync

[Jaffacakelover]

RetroPie guides say to connect a USB device containing your content, which will then sync to your Pi. Guides then say to “wait a few minutes for the USB drive's indicator light to stop flashing”. Firstly this is rather vague, and secondly I had to dig out a USB stick that had a light on it (many don't)!

I'd suggest adding a status popup to explain what's going on, complete with loading animation (but no progress bar / percentage), providing feedback to those with or without an LED in their flash drive.

Syncing files from USB Device...

The message could appear along the top of the screen:

Or it could appear bottom right, if space permits on lower resolutions:

Or it could even be a toast popup:

It would then display a message like this when finished:
Sync complete: Restart Emulationstation to see changes.