Improve card draggability when behind the dock

[clpetersonucf]

The "dock" element in Sort It Out takes z-axis priority over the "desktop" which contains the draggable cards. Since they occupy different stacking contexts, the cards remain under the dock despite having higher z-index values. This means it's possible to drag a card into the space around a folder and be unable to pick it up again. Resolving this issue ought to resolve #55 as well.