startkde: Forward stdout/stderr of started processes

QProcess::start defaults to capturing all output, but that got lost as it is never read. Set QProcess::ForwardedChannels explicitly where it was missing, so that output ends up in the correct place again.

Downstream report: https://bugzilla.opensuse.org/show_bug.cgi?id=1196206

This issue might've also caused excessive memory use as the output was collected in memory buffers.