Provides tab functionality.
Creates Tab object. The element specifies a container where tab contents will be placed.
Adds a new clicker - window pair, i.e. when user click clicker, window will be shown.
Switches to tab, according to index.
var t = new Tab("content");
t.add("header_1","window_1");
t.add("header_2","window_2");
t.go(0);