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)

Demos3/ImportURDFDemo/ImportURDFSetup.h

@@ -6,12 +6,16 @@
 
 class ImportUrdfDemo : public CommonMultiBodySetup
 {
+    char m_fileName[1024];
+    
 public:
     ImportUrdfDemo();
     virtual ~ImportUrdfDemo();
 
 	virtual void initPhysics(GraphicsPhysicsBridge& gfxBridge);
 	virtual void stepSimulation(float deltaTime);
+    
+    void setFileName(const char* urdfFileName);
 };
 
 #endif //IMPORT_URDF_SETUP_H