add fileOpenDialog and enable loading of urdf from GUI

(will add .bullet file support soon)
Uses native Windows (getFileOpenFileName) and Mac OSX NSOpenPanel,
on Linux using pipe popen to zenity)

btgui/OpenGLWindow/MacOpenGLWindow.h

@@ -81,7 +81,7 @@ public:
 	
 	virtual void setWindowTitle(const char* title);
 
-	
+    int fileOpenDialog(char* filename, int maxNameLength);
 
 };