Systemd integration for XDG and Qt

[Mikilio]

Description

Options for XDG and Qt are successfully setting home.sessionVariables but never systemd.user.sessionVariables.
This causes a lot of bugs and it's not obvious that one even needs to set systemd.user.sessionVariables. I'd say that all environment variables set in the respective modules should also be set if qt.systemdIntegration/xdg.systemdIntegration are enabled.

EDIT: I was wrong about xdg, however it seems that certain services started by dbus without systemd do not receive environment variables like XDG_DATA_DIR either.

[stale]

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