BCheckBox: Add tri-state support

* Add possible control state B_CONTROL_PARTIALLY_ON and support it in
  BCheckBox and BControlLook.
* BCheckBox: Add partialStateToOff property defining whether the
  partial state should transition to off. Defaults to false (i.e.
  partial to on).

headers/os/interface/Control.h

@@ -12,7 +12,8 @@
 
 enum {
 	B_CONTROL_OFF = 0,
-	B_CONTROL_ON = 1
+	B_CONTROL_ON = 1,
+	B_CONTROL_PARTIALLY_ON = 2
 };
 
 class BWindow;