Implement tabbed WCI containers

Important: The Window Container Interface is a desktop application feature (for GDC), and is not supported on other front-ends (web and mobile).

WCI container can also display the child programs in a folder tab, when the presentation style attribute tabbedContainer is set to yes.

With a tabbed window container, the style attribute tabbedContainerCloseMethod defines how to close the current page.

Values can be:

"container" (default), the container has a close button on the top right corner, which closes the current tab.
"page", each page has its own close button.
"both", each page and the container have a close button.
"none", no close button is shown.

The close button is enabled depending on the window style attribute.