Remove widget values.

* WWidget no longer stores a value independent of its connected controls.
* Make a connection explicitly the display value. For legacy reasons this is the
  last connection with connectValueToWidget.
* Update all WWidget descendents to use the connected display value when
  painting.
* Push buttons update the left or right connected control for push-button
  behavior or toggling.

This allows WPushButton to toggle a multi-state button on left and right
click. This was previously impossible.