[escort] Support multiple tabs

WindowImp now supports DeskTop and TopLevel modes.  In DeskTop mode, when a
top-level browsing context is created, a new iframe element is created
in the DeskTop window for the new top-level browsing context.

The revised escort.html captures the DOMNodeInserted events of iframe
elements created in the DeskTop window, and lay out iframe elements like
tabs.

data/escort/escort.html : Add the tab bar and add/refine support functions;
(WindowImp) : Support DeskTop and TopLevel modes.
(WindowImp::WindowImp) : Take another argument to specify the window mode; all callers changed
(WindowImp::open, WindowImp::getTop, WindowImp::getParent) : Refine
(WindowImp::selectBrowsingContext, WindowImp::navigate): New
(DocumentImp::createElement) : Support DeskTop and TopLevel modes.
(HTMLIFrameElementImp::HTMLIFrameElementImp) : Refine
(HTMLAnchorElementImp::handleClick) : Check the target attribute