Attempts to close the window.
Returns S_OK if successful, or an error value otherwise.
IWindowProvider::Close must return immediately without blocking.
IWindowProvider::Close raises the WindowClosedEvent (Window_WindowClosed_Event_GUID) event. If possible, the event should be raised after the control has completed its associated action.
When called on a split pane control, this method will close the pane and remove the associated split.
This method may also close all other panes depending on implementation.