[NullsoftInstallerPackager] Add a way to set the working directory of shortcuts

Previously, the working directory has always been set to $INSTDIR. Some applications (such as Kate) set the initial location of file pickers to the working directory, and the installation directory is not so useful in that regard.

The case with Kate on windows (Merge Request) is my main use case for this patch, but I think it can still be useful in the future.

I have only tested this with kdeconnect-kde in windows as that's the only environment I have access to for the time being. I hope this patch does not introduce regressions!