startkde: Forward stdout/stderr of started processes

Fabian Vogt requested to merge work/fvogt/startforward into master

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:

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

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