Show/hide panel shadow

concrete5 currently add a `<div class="ccm-panel-shadow-layer">` element on the right side of the left panel to add a faint shadow on the right.
BTW, this element overlaps the scrollbar in case the panel is tall, and scrolling the panel can be done only with the mouse wheel since the scrollbar is not interactive.
So, let's hide the left panel shadow if the scrollbar is visible.