dijit.layout.TabContainer close icon disappears

[dante]

when you hover a close icon, the node disappears, shifting layout, and other weirdness. cannot close tabs. see dijit/tests/layout/test_TabContainer.html

[bill]

(In [10706]) Refactor setStateClass() related code so that:

• baseClass is always specified in javascript, in this.baseClass, not on the dom node
• If you need different CSS classes based on (for example) hovering over the tab label vs. the tab close button, set stateModifier="CloseButton?" on the tab close button node (which must also have dojoAttachEvent="onmouseenter: _onMouse, onmouseleave: _onMouse"

Fixes #4609, #4211, and I suppose it also allows the user to affect styling by specifying baseClass=foo in markup. Well, that was working before for most of the form widgets but I guess now it works for all of them.

[bill]

(In [10723]) sometimes event.target is the document rather than a node. refs #4609.