Do not show empty ad slots.

[cobbdb]

If a slot has already had googletag.display called on it, then it should be exempt from future calls to show slot/breakpoint.

This should only apply to empty slots as filled slots may be legitimately shown/hidden. However empty slots will never want to be shown - only hidden.

There should also be a force setting to specifically override this default behavior and show an empty ad slot.

[cobbdb]

Problem is:

• show on slot => empty ad call => collapseEmptyDivs hides slot
• hide on slot => noop
• show on slot => display:block on empty slot

[cobbdb]

Breaking change as some applications may rely on this behavior.

[cobbdb]

Show/Hide has always been an awkward and presumptuous part of Harmony. Instead of fixing this, I'm removing hide.slot() and hide.group() as well as show's display:block from version 6.