show correct interleaving of stdout and stderr messages

In the application output messages to stderr and stdout are cached separately, and thus their correct interleaving is often lost.
To ensure the correct interleaving an option to merge stderr and stdout has been added, this looses some highlighting.
Added a patch to fix it on *nix systems. With the current setup stderr is not directly used on windows, so the option to merge them does not make sense, and the option is hidden.
Ossi is working in changing windows so that merging would make sense also there. Holding the change for it.