Workaround if loading themed icon via filename with extension is not supported by default

In cases where _xfdashboard_image_content_load_from_icon_name() gets a filename with extension, e.g. "skype.png", and gtk_icon_theme_lookup_icon() fails to lookup icon then fallback to lookup icon without extension. But before doing this lookup we check if gdk-pixbuf supports this file extension at any of its formats.