[wayland] Pass command line arguments to launched processes

Summary:
Since QProcess::setProgram() doesn't split the command, we need to do it
ourselves.

Test Plan: `dbus-run-session kwin_wayland "kate foo.txt"` works again.

Reviewers: #kwin, cblack, davidedmundson, apol

Reviewed By: #kwin, cblack, davidedmundson, apol

Subscribers: apol, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29253