Introduce ISO List dialog
    
    The motivation for this dialog started with rhbz #1310624, where it was
    reported that foreign menu was causing too many debug messages to be
    printed to the console, because remote viewer had a timeout of 5 seconds
    to refresh the ISO list automatically.
    
    As a workaround, the timeout was adjusted for 5 minutes, but it could
    cause more problems, such as inconsistencies between what was shown by
    remote viewer and what the server had configured.
    
    Another issue caused by displaying the ISO files as a menu item was that
    if the list was too long, it would take all the available space on the
    screen. In the end, a menu item was not the correct choice of UI
    component for this use case.
    
    In order to solve both problems, we now present the ISO list as a
    dedicated dialog, where the refresh of ISO list is triggered manually by
    the user and the list is contained within the dialog, by displaying de
    files in a treeview.
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>