Introduce new xwiki-skin component and build module

    public VelocityEngine getVelocityEngine() throws XWikiVelocityException
    {
        // Get the current skin in order to find the corresponding VelocityEngine instance.
        String skin = this.skinManager.getSkin();

        // Note: It's possible that several threads call this method at the same time and thus
        // it's possible that a few instances for the VelocityEngine are created. However
        // this is not a real problem even though it'll use a little bit more memory.
        // The alternative which is to make the method synchronized would penalize performances
        // which is why we're not synchronizing it.
        
        VelocityEngine engine;
        if (this.velocityManagers.containsKey(skin)) {
            engine = this.velocityManagers.get(skin);
        } else {
            String skinMacros = this.skinManager.getSkinResource("macros.vm");
            Properties properties = new Properties();
            String macroList = this.skinManager.getTemplateResource("macros.vm") 
                + ((skinMacros == null) ? "" : "," + skinMacros); 
            properties.put(RuntimeConstants.VM_LIBRARY, macroList);

            try {
                engine = (VelocityEngine) this.componentManager.lookup(VelocityEngine.ROLE);
            } catch (ComponentLookupException e) {
                throw new XWikiVelocityException("Failed to create Velocity Engine", e);
            }
            engine.initialize(properties);
            this.velocityManagers.put(skin, engine);
        }

        return engine;
    }