VirtViewerApp: create main window after constructor
    
    When using the configuration file to specify which remote monitors
    should be enabled when using the --full-screen option, it sometimes left
    additional displays enabled, or didn't place the displays on the right
    monitor, or didn't fullscreen them.
    
    Part of the problem was that we were creating the first display window before
    loading the monitor mapping configuration from the settings file. So even if
    the first display was disabled in the configuration, the first window will
    still be created with an id of 0, and therefore didn't get set to fullscreen.
    Moving the main window creation to the 'constructor' vfunc instead of the
    object init func ensures that the configuration is all loaded before we attempt
    to do any fullscreen autoconf.
    
    Related: rhbz#1200750