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: Ia18b29b95e77d92a2d1602cd77b027aa56047962
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>