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/AllBullet2Demos/main.cpp

@@ -181,6 +181,32 @@ static void MyMouseButtonCallback(int button, int state, float x, float y)
 
 #include <string.h>
 
+void openURDFDemo(const char* filename)
+{
+   
+    if (sCurrentDemo)
+    {
+        sCurrentDemo->exitPhysics();
+        app->m_instancingRenderer->removeAllInstances();
+        delete sCurrentDemo;
+        sCurrentDemo=0;
+    }
+    
+    app->m_parameterInterface->removeAllParameters();
+   
+    ImportUrdfDemo* physicsSetup = new ImportUrdfDemo();
+    physicsSetup->setFileName(filename);
+    
+    sCurrentDemo = new BasicDemo(app, physicsSetup);
+    
+    if (sCurrentDemo)
+    {
+        sCurrentDemo->initPhysics();
+    }
+
+    
+}
+
 void selectDemo(int demoIndex)
 {
 	sCurrentDemoIndex = demoIndex;
@@ -329,6 +355,18 @@ struct GL3TexLoader : public MyTextureLoader
 	}
 };
 
+void fileOpenCallback()
+{
+
+ char filename[1024];
+ int len = app->m_window->fileOpenDialog(filename,1024);
+ if (len)
+ {
+     //todo(erwincoumans) check if it is actually URDF
+     //printf("file open:%s\n", filename);
+     openURDFDemo(filename);
+ }
+}
 
 extern float shadowMapWorldSize;
 int main(int argc, char* argv[])
@@ -476,7 +514,8 @@ int main(int argc, char* argv[])
 	*/
 	unsigned long int	prevTimeInMicroseconds = clock.getTimeMicroseconds();
 
-
+    gui->registerFileOpenCallback(fileOpenCallback);
+    
 	do
 	{