BBox: Always offset the top border by the same amount

...so that the top border of BBox's with no labels, BBox's with
text labels, and BBox's with BControl labels will all line up.

headers/os/interface/Box.h

@@ -63,6 +63,7 @@ class BBox : public BView {
 		virtual	void		ResizeToPreferred();
 		virtual	void		GetPreferredSize(float* _width, float* _height);
 		virtual	void		MakeFocus(bool focused = true);
+		virtual	void		SetFont(const BFont* font, uint32 mask = B_FONT_ALL);
 		virtual	status_t	GetSupportedSuites(BMessage* message);
 
 		virtual	status_t	Perform(perform_code d, void* arg);
@@ -96,6 +97,7 @@ class BBox : public BView {
 		border_style		fStyle;
 		BView*				fLabelView;
 		LayoutData*			fLayoutData;
+		float				fLabelHeight;
 };
 
 #endif	// _BOX_H