Widget: Improve API for direct instantiation

When instantiating a widget directly (new $.ui.dialog( options, element )), the following should be improved:

• The new keyword should be optional
• When not using the new keyword, the widget should be initialized even if called with no parameters
• When called with the new keyword, the widget should not be initialized to allow the simple inheritance pattern to continue to work
• The widget should be able to generate an element if one is not provided
• Providing a selector or jQuery object instead of a DOMElement should fully work
• Currently the widget will be instantiated and this.element will be correct, but the instance will not be stored as data on the element