package: Add appdata metadata for Gnome Software
    
    Adds metadata to be used with Gnome Software.
    Includes name, summary, description and a few screenshots of remote-viewer.
    
    Signed-off-by: Lukáš Venhoda <lvenhoda@redhat.com>
    Acked-by: Daniel P. Berrange <berrange@redhat.com>
    Acked-by: Fabiano Fidêncio <fidencio@redhat.com>