commandlauncherjob: Quote command to prevent truncation

When executing a shell script or an executable without this patch in dolphin in a folder with a blank e.g. "/my test/test.sh" the path is truncated to "/my" and the command is not executed.

Bug: 423412

Please not: The "startProcessAsCommand" tests will start to fail, those tests use a string with command+args construct instead of just a single command. Arguably with the command+args construct, one ought to use the other interface: CommandLauncherJob(const QString &executable, const QStringList &args, QObject *parent) instead of the one changed here: CommandLauncherJob(const QString &command, QObject *parent)