Bug 7466 - Cart notification popup should appear onscreen even when button isn't

Bug 7466 - Cart notification popup should appear onscreen even when button isn't

This patch compares the scroll position of the window with the page
position of the cart button and displays the cart notification
popup at the top of the visible page if the cart button has moved
offscreen.

Includes fix for OPAC and staff client.

Signed-off-by: Aleksa Vujicic <aleksa@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
(cherry picked from commit 1eb0cbc5bf728f6cdb64b5cf102f227df1efb617)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>