Trebuchet: Fix stack effect

Actually stack effect is broken. Dragging an item from the current page to other fails, because
the page has an alpha value of 0, and the dragged icon falls into a hidden state.

Patchset 2: Fixed the overlap of views of stack effect on tablets

Change-Id: I7c607eee29fbd82542b49d2c6bd50d7554e6d673
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>