At the moment videos without a description are not shown - setting
default content to "" fixes this.
Another current bug is that thumbnails are not displayed. This is caused
by a double slash in the url. For this every trailing slash is now
stripped (for backwards compatibility) and the API response is correctly
parsed.
The language_support variable is set to True by default,
and set to False in only 5 engines.
Except the documentation and the /config URL, this variable is not used.
This commit remove the variable definition in the engines, and
set value according to supported_languages length: False when the length is 0,
True otherwise.
Close#2485
