Support <footer> element in window component

The window component in ZK gives a lot of customization options for the title section, providing buttons, title and caption support.

But a frequent requirement is to include buttons as a window footer (see Jira "Create Issue" dialog for example).

It would be a great enhancement to support a <footer> element as the last child of a <window> element to make it easy to include this kind of functionality and not to be forced to rely on <borderlayout> and <south> components.