Bug#656803: libgtk-3-doc: image files in reference manual are missing

[Rafal Dabrowa]

Not only images are missing. Some html files instead of code snippets 
have only a message:

     FIXME: MISSING XINCLUDE CONTENT

See for example:
     gdk3/gdk3-Windows.html
     gtk3/gtk-getting-started.html
     gtk3/GtkApplication.html

Rafal