Issue description

IMHO this is the most notable and irritating bug in Nix/NixOS. Basically, if there are multiple versions of the same QT plugin in /nix/store, all QT applications which use that plugin will fail with "QT library version mismatch" error. I am wondering if there is a solution which doesn't require deleting older generations and lets you update the system regularly.

Steps to reproduce

Install multiple versions of QT;
Launch some QT-based applications.