DBus activated services such as mako use the XCURSOR_THEME and
XCURSOR_SIZE environment variables to decide how to show the cursor,
so without these, the cursor may not match the rest of the desktop
when hovering over (in this example) mako notification surfaces.
Without this, even if you configure a preference for Electron apps to
use Ozone by setting `NIXOS_OZONE_WL=1`, GUI apps launched through
systemd user services use XWayland, since the variable is not set in
their environment.
This fixes that issue by importing it, like we do other variables.
- The `XDG_SESSION_TYPE` environment variable is used by some applications and frameworks to
detect wayland sessions (i.e qt5/6, electron/chromium). It is set by wlroots since version 0.13.0 [1].
- Propagating `XDG_SESSION_TYPE` to the systemd user environment is necessary when processes launched by
services (e.g emacs) need to inherit the environment variable.
[1] - 90c8452959
When setting `...sway.package = null`, the default bar configuration
would throw an error trying to use the bar from the null package.
Logic is added to use the bar from `pkgs.sway` instead of `cfg.package`
if it is null.
Fixes#1714
