Make click-drag cursor in overview a move cursor

JAL-2611 introduced new functionality to the overview window where if the user clicks inside the overview window's red box, they can drag the box to a new position (whereas previously the box would have been positioned at the click point). In JAL-2611 the cursor was left as the default cursor, but it should be a move cursor (crosshair with double-headed arrows). Unfortunately on Macs the predefined move cursor is the same as the default cursor, so a custom cursor is required.