Fix the bounding box of message boxes

Multi-line message boxes had the incorrect height.

I also took the opportunity to make named constants out of magic numbers
making up the various message box dimensions.

src/font.h

@@ -23,6 +23,7 @@ public:
 	~Font();
 
 	int getTextWidth(const std::string& text);
+	int getTextHeight(const std::string& text);
 
 	int getLineSpacing()
 	{