FIX Multiple redraw calls on navigation
FIX Dont update preview iframe if hidden

Updating the iframe src when the iframe isnt visible in IE8 causes a
view disconcerting font glitch (and it slows down navigation anyway),
so if the iframe isnt visible, delay setting the src until it is
Merge pull request #2345 from hafriedlander/fix/performance

Couple of performance fixes - eliminate un-necessary redraws and preview updating