Start re-organizing demos so the physics setup can be shared easier (explicit create graphics objects, init/exit physics etc)

Add B3G_RETURN key code, only implemented in Windows so far (todo: Mac, Linux)
Fix Windows key management (use WM_CHAR event instead of WM_KEYUP
Add Return (OnKeyReturn) key support TreeNode, so we can select an item using the return key.

btgui/OpenGLWindow/b3gWindowInterface.h

@@ -39,7 +39,8 @@ enum {
 	B3G_BACKSPACE,
 	B3G_SHIFT,
 	B3G_CONTROL,
-	B3G_ALT
+	B3G_ALT,
+	B3G_RETURN
 };
 
 struct b3gWindowConstructionInfo