qt5ct: add a nixos module to enable qt5ct

In order to use qt5ct (Qt5 Configuration Tool) to configure Qt5
settings (theme, font, icons, etc.) under DE/WM without Qt
integration, the environment variable QT_QPA_PLATFORMTHEME should be
set to "qt5ct".

It can be done automatically by this module, or by setting the
variable explicitly in the user or in the system configuration.