Document button rewrite, buttonset replacement controlgroup and new checkboxradio widget

[jzaefferer]

• Button: Different markup, different options, but no more support for radio and checkbox buttons, theming
• Buttonset is gone, replace by controlgroup, with a different API and support for non-button widgets
• New checkboxradio widget, API somewhat similar to button

[jzaefferer]

Note controlgroup items option has a "spinner" property, with a class selector as the value. Requires either adding the class manually or initializing the spinner manually. Can be overridden to use input[type=number], but that also requires custom CSS to remove the native number controls.

[jzaefferer]

Fixed in 1-12 branch.