fix quoting in AsyncWgetFileGetter

make youtube file wfAnJt-j4Gs work, it has a funny title with lots of special characters:
►Heinrichs & Hirtenfellner @ Licht & Liebe Berlin Opening ♬♫ ..•❤* ★¨`•♫.•´.¸.•´❤♫♬♩

- use ' instead of " to escape shell arguments
- use qq{} instead of "" for more readable perl code
- overwrite FileGetter::_download() instead of duplicating all of FileGetter::get()

Conflicts:
	videosite/AsyncWgetFileGetter.pm