Working in multiple devices on 2015 is great, however sorting content becomes a bit tricky. Would it be possible to add a check box when you are uploading content that specifies if the content is for tablet or phone?
This would allow the user to sort by device when they are in this view:
Yes, there is a degree of "you gotta know what you've uploaded" to get the right keyword in place. But then you'd need to do the same thing if we were to add the check box you're proposing...
Maybe not "perfect" -- but maybe it's a way to address what you want today. 🙂
-- Details --
When you create an article (and banner) you can add internal keywords, and you can add them by editing the item's properties. Then use the filter (keywords) -- This would let me filter to only show content with the internal keyword of "phone" or "tablet" -- I cannot sort on this value, but maybe filter would isolate what you want to work with.
(don't forget about batch editing if you need to add an internal keyword to a bunch or articles)