Tab hover class doesn't work for dynamically added tabs

Mouse hover handlers are only binded for static tabs (which were already present when tabs() was called). However, for tabs added using tabs("add", ...), those event handlers are never bound, so their appearance doesn't change when mouse-hovered.

I traced the problem down to _tabify() being called without the 'init' argument in the add() function. The code which binds the events expects 'init' to be defined, but I don't understand the logic well enough to fix this myself.