Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.
Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.
This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.
Fixes#1906
This target is for systemd units that require a system tray to be
running.
This also fixes taffybar.service: previously, systemd would consider it
to be active (running) before it was actually ready to accept tray
icons.
