Abstracted out the window management
Added the host implementation of :e command

It is now possible to open files with the :e command.  The :split
command appears broken but that seems unrelated to this.
Fixed the :split command

The building up of the Content portion of the TabItem wasn't first
clearing the previous content.  This meant that the
IWpfTextViewHost.Control value still had a logical parent and hence
adding it to the new grid was causing an exception .