bug#74197: 29.4; Newsticker feed headline appears duplicated

[Ulf Jasper]

The observed behaviour occurs, when

- newsticker is told to display the feed logo
  (i.e. 'newsticker-heading-format' contains '%l' (or '%L')), and

- newsticker is not able to display the feed logo (e.g. Emacs is running
  in a terminal due to arg '-nw')

In that case, instead of the feed logo, the feed name appears in the
newsticker buffer.

Newsticker will now insert "[logo: <feed name>]" instead of the bare
feed name in order to mark it as a replacement for the actual logo
image, like this:

    [logo: BBC Weather - Forecast for  London, GB]
    BBC Weather - Forecast for  London, GB (Mittwoch, 13:31) [3 + 0 + 0 + 0 = 3]
    Today: Light Cloud, Minimum Temperature: 10°C (51°F) Maximum Temperature: 
13°C (55°F) (Mittwoch, 12:00)
    (Mittwoch, 12:00) Maximum Temperature: 13°C (55°F), Minimum Temperature: 
10°C (51°F), Wind Direction: North Easterly, Wind Speed: 3mph, Visibility: 
Moderate, Pressure: 1031mb, Humidity: 89%, UV Risk: 1, Pollution: Low, Sunrise: 
07:04 GMT, Sunset: 16:24 GMT