Implemented _ToggleButton base class (extended from Button), and made PageButton and TabButton (internal widgets used by PageController and TabController) extend _ToggleButton.
Implemented _ToggleButton base class (extended from Button), and made
PageButton and TabButton (internal widgets used by PageController and TabController)
extend _ToggleButton.

Leverages FormElement._onMouse() to do all the CSS manipulations.  _onMouse() was
enhanced to understand the idea of toggle controls, that stay selected
(dijitFooActive CSS class) until programatically released.

Fixes #3238.

git-svn-id: 674a2d6b1e32e53f607eb824547723f32280967e@8958 560b804f-0ae3-0310-86f3-f6aa0a117693

Sam-Cummins/delite,kitsonk/delite,Sam-Cummins/delite,kitsonk/delite,harbalk/delite,harbalk/delite