FormElement generalized code for mouse and CSS class management

[bill]

Need library code to handle mouse events, setting appropriate flags on the widget:

• hover - mouse is over the widget
• active - mouse button is being pressed down

Widget also has other flags that aren't directly related to mouse movement:

• disabled
• selected - if a checkbox is checked, or a tab is selected, etc.

Also need a function to set a single CSS class for the widget depending on the state variables above.

Classes for button:

• buttonDisabled
• buttonHover
• buttonActive

Classes for checkbox with no check mark:

• checkboxDisabled
• checkboxHover - mouse over checkbox but not pressing buttons
• checkboxActive - between mouse down and mouse up

and parallel classes for when the box is checked:

• checkboxSelectedDisabled
• checkboxSelectedHover
• checkboxSelectedActive

A widget like Combobox needs to have separate CSS class names for when the mouse is over the input vs when it's over the down arrow.

• comboboxDisabled
• comboboxHover - mouse over checkbox but not pressing buttons
• comboboxActive - between mouse down and mouse up

and parallel classes for when the box is checked:

• comboboxDownArrowHover
• checkboxDownArrowActive

Still talking w/Owen to finalize this plan.

[bill]

(In [9008]) Refactor Checkbox and RadioButton? to take advantage of dijit_a11y flag. Since we now know if we are in high-contrast mode of not, no need to convert from background-image to <img>.

Still have to do some more refactoring after FormElement?._onMouse (refs #3350) is complete.

Fixes #3351.

[bill]

(In [9024]) Start refactor of _onMouse() as outlined in ​http://trac.dojotoolkit.org/ticket/3350. Refs #3350 and fixes #3359. Tundra's ToggleButton? needs some CSS love; hard to differentiate the two states.

[bill]

(In [9046]) Refactor Checkbox/Radio? to extend ToggleButton?. Refs #3350.