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.
Assert ControlWidgetConnection::m_pControl is non-NULL.
Rename ControlWidgetConnection methods to be more clear.
Add comment to left-click push button handling.
Rename methods for getting and setting connected control parameters to be more clear.
Delete widget connections when the widget is deleted.
Style misc.
Wrong destructor :-/.
Rename reset() to resetControl().
Add parentheses for clarity.
Merge pull request #146 from rryan/basewidget

Remove widget values.