Add zsh completions (compdef) for plasmawindowed
Taking this chance to factor out plasma_packages completions after all. It may be quite specific, but it's already needed in 3 places.
Technically, plasmawindowed can take any number of arguments, not just 1. But I'm having hard times explaining it to zsh _arguments. The trick that worked with -e
option for konsole isn't applicable for positional arguments, it seems.