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/CheckBox.h

@@ -69,6 +69,9 @@ public:
 
 	virtual	status_t			Perform(perform_code code, void* data);
 
+			bool				IsPartialStateToOff() const;
+			void				SetPartialStateToOff(bool partialToOff);
+
 protected:
 	virtual	void				LayoutInvalidated(bool descendants = false);
 
@@ -81,6 +84,7 @@ private:
 private:
 			BRect				_CheckBoxFrame() const;
 			BSize				_ValidatePreferredSize();
+			int32				_NextState() const;
 
 private:
 	// Forbidden
@@ -89,6 +93,7 @@ private:
 private:
 			BSize				fPreferredSize;
 			bool				fOutlined;
+			bool				fPartialToOff;
 };
 
 #endif // _CHECK_BOX_H

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;

headers/os/interface/ControlLook.h

@@ -69,13 +69,14 @@ public:
 	};
 
 	enum {
-		B_FOCUSED			= 1 << 0,
-		B_CLICKED			= 1 << 1, // some controls activate on mouse up
-		B_ACTIVATED			= 1 << 2,
-		B_HOVER				= 1 << 3,
-		B_DISABLED			= 1 << 4,
-		B_DEFAULT_BUTTON	= 1 << 5,
-		B_IGNORE_OUTLINE	= 1 << 6,
+		B_FOCUSED				= 1 << 0,
+		B_CLICKED				= 1 << 1, // some controls activate on mouse up
+		B_ACTIVATED				= 1 << 2,
+		B_HOVER					= 1 << 3,
+		B_DISABLED				= 1 << 4,
+		B_DEFAULT_BUTTON		= 1 << 5,
+		B_IGNORE_OUTLINE		= 1 << 6,
+		B_PARTIALLY_ACTIVATED	= 1 << 7, // like B_ACTIVATED, but for tri-state
 
 		B_BLEND_FRAME		= 1 << 16
 	};