Add method which parses command from a string

[DvdGiessen]

To make it easier to execute a command line given as a string without first having to split the string in the calling application, we could add a method that parses a string into a command, arguments, and perhaps even environment variables, similar to how the shell would parse such a command line.

The shellwords package seems to be perfect for this.

This will also help work around the issue noted in #2, namely that there's no easy way to call a DBus method with a StringList argument in the KWin JS API.

[r0skar]

I think that makes a lot of sense and (imo) makes the behavior more intuitive. Even the naming of the actions (Exec.Cmd) suggest any "valid" command should work.