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/Win32OpenGLWindow.h

@@ -51,6 +51,8 @@ public:
 	virtual	void	endRendering();
 
 	virtual float getRetinaScale() const {return 1.f;}
+
+	virtual int fileOpenDialog(char* fileName, int maxFileNameLength);
 };