app: Check validity of hotkey
    
    The hotkey is valid if it has a valid value. The value is valid if it is
    not empty and is successfully parsed by gtk_accelerator_parse().
    
    These hotkeys formats are considered invalid:
     "key" - missing value
     "key=" - missing value
     "key=abcd" - value cannot be parsed by gtk_accelerator_parse()
    
    Resolves: rhbz#1339572
    
    Acked-by: Fabiano Fidêncio <fidencio@redhat.com>