Fixed the top navigation mainlevel counter.

Fixed the top navigation mainlevel counter.

It was not resetting when a new menu was called due to the nature
of the static variable being used to keep track of how many main
level items have been iterated ($mainLevelCounter)

The solution was another static var ($topbar_id_old) comparing the
old topbar id to the current topbar id (derived from $args), and if
they differ reset the mainlevel counter.

This avoids the problem where the mainlevelcounter starts counting
up in the Top Links, causing the Drop-left class to be improperly
applied (since that looks at $mainLevelCounter < 3 but by the time
it gets there, if a Top Links menu exists, it's already at 2 or 3)