Provide a way to render pages partially

Provide a way for pages to be partially rendered. It should be possible to render any component, including all child components.

By default, components will be 'standalone', meaning that only their doBeforeRender method will be invoked when a partial render request is handled. It should be possible for components to be marked 'non-standalone', meaning that they do use additional request attributes, provided by other components. If such a component is rendered, all components on the page should be invoked in their doBeforeRender method.

Components can determine from the request (context) whether the request is for partial rendering and whether they will be rendered themselves. This is for optimization purposes, allowing components that would normally do complex operations to skip those if they do not affect the rendered result.