Send notification via GioApplication, not ashpd

Using ashpd I get `Invalid client serial` too many times.
At least now the notifications keep showing, even if ashpd
is still used to monitor network changes, which could still
stop working arbitrarily after `Invalid client serial`

ntfy-daemon/src/models.rs

@@ -261,3 +261,12 @@ impl From<Status> for u8 {
         }
     }
 }
+
+pub struct Notification {
+    pub title: String,
+    pub body: String,
+}
+
+pub trait NotificationProxy: Sync + Send {
+    fn send(&self, n: Notification) -> anyhow::Result<()>;
+}