Multitasking cards: Drop labels

Now that we have proper window thumbnails, we no longer need the label stopgap at the bottom to have windows be distinguishable. Instead, we can move the app icon to the center and have it overlap the bottom edge, like in the mockups:

Since the thumbnails in the overview are currently still quite a bit smaller than in the mockups, I'd go with 32px icons (rather than 64) for now.