Replaces notify-rust with directly calling into the zbus crate.
This provides a pure rust interface to DBUS and provides more
flexible control over notification handling.
closes: #485
This commit implements the deprecated NSUserNotification bits needed
to be able to handle clicking on a notification and open our choice
of URL.
Ideally we'd use the newer UserNotifications framework, but that
requires code signing.
* Allow injecting some initial output to new panes
* Have the update checker set this new-pane-banner to a short
upsell to let the user know there is an update.
* Refactor toast notifications into their own crate
* Have the update checker call a new stub function that triggers
a toast notification with an URL... but it does nothing because
the rust ecosystem doesn't support this on macos yet and I'm
writing this code there
