Eleven: Make the square widget a true square

This brings back the small artwork size that was apparently accidentally
removed during the last UI overhaul. While at it, adjust the padding and
the play/pause button size to make everything fit in the actually listed
two-by-two area without being unevenly weighted (at least on the default
display scale or smaller).

Also, rename the widget to "Square", as "4x1" is no longer accurate.

Change-Id: If66348f19c5a6025cd4a8a059178b46dcc1c8964