css: Immediately transition visibility when showing popup
css: Immediately transition visibility when showing popup

For reasons I don't understand, an activated popup was visible during
transition even though the boolean `visibility` property didn't switch
to `visible` until the end of the 0.3s transition. This prevented
input elements from getting focus until the end of the transition. Now
input elements can get focus right away.

ether/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite