ToggleSplitter doesnt provide any convenient way to get open/close notification

[Sam Foster]

Open/closing are what the widget does, but its doesnt provide any topic or readily connect-able method for catching that event

[Sam Foster]

(In [15121]) * Adds onOpen, onClose stubs that are called when there's a state change. Added example to test page

• Refactored slightly to use this.attr to call the _setOpenAttr always, lose the 2nd "force" param.

As a result attr("open") calls are ignored until the end of startup, when a this._started flag is switched.

• tweaks for code style
• css tweaks - the -1px offset on the button not necessary

fixes #7590