move 'main' functions into a separate folder, to make it easier to create standalone demos (console app without gui, or opengl app, and possibly even other versions, like a software renderer, producing pictures in the cloud)

Add a line in an example cpp file to allow a standalone version.
See examples/BasicExample.cpp: B3_STANDALONE_EXAMPLE(BasicExampleCreateFunc)

examples/StandaloneMain/main_opengl_single_example.cpp

@@ -0,0 +1,65 @@
+/*
+Bullet Continuous Collision Detection and Physics Library
+Copyright (c) 2015 Google Inc. http://bulletphysics.org
+
+This software is provided 'as-is', without any express or implied warranty.
+In no event will the authors be held liable for any damages arising from the use of this software.
+Permission is granted to anyone to use this software for any purpose, 
+including commercial applications, and to alter it and redistribute it freely, 
+subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+*/
+
+
+
+#include "../CommonInterfaces/CommonExampleInterface.h"
+#include "../CommonInterfaces/CommonGUIHelperInterface.h"
+#include "BulletCollision/CollisionDispatch/btCollisionObject.h"
+#include "BulletCollision/CollisionShapes/btCollisionShape.h"
+#include "BulletDynamics/Dynamics/btDiscreteDynamicsWorld.h"
+
+
+#include "LinearMath/btTransform.h"
+#include "LinearMath/btHashMap.h"
+
+
+
+#include "../OpenGLWindow/SimpleOpenGL3App.h"
+#include <stdio.h>
+#include "../ExampleBrowser/OpenGLGuiHelper.h"
+
+
+int main(int argc, char* argv[])
+{
+
+	SimpleOpenGL3App* app = new SimpleOpenGL3App("Bullet Standalone Example",1024,768,true);
+	
+	OpenGLGuiHelper gui(app,false);
+    
+	CommonExampleOptions options(&gui);
+	CommonExampleInterface*    example = StandaloneExampleCreateFunc(options);
+        
+	example->initPhysics();
+
+	do
+	{
+		app->m_instancingRenderer->init();
+        app->m_instancingRenderer->updateCamera();
+
+		example->stepSimulation(1./60.);
+	  	
+		example->renderScene();
+ 	
+		app->drawGrid();
+		app->swapBuffer();
+	} while (!app->m_window->requestedExit());
+
+	example->exitPhysics();
+	delete example;
+	delete app;
+	return 0;
+}
+