The ParseQuoteWord function from APT's strutl's was doing way less magic than I thought. It is much easier, safer and less error-prone to use the wonderful QString API to parse the selections list.

I also made it so that it only attempts to mark uninstalled packages for installation.
This results in a great speedup on enormous lists, but has the side effect of not upgrading installed packages.
I'm not sure if that's desired anyways, so it's a great tradeoff.