status-area: Call AboutToShow before showing menu

Comments in `gnome-shell-extension-appindicator` indicate this is
necessary for some clients. Checking the return value doesn't seem to
be.

There are more comments about Dropbox; more may be needed to fix it,
which doesn't seem to behave very properly.

cosmic-applet-status-area/src/subscriptions/status_notifier_item.rs

@@ -235,6 +235,8 @@ trait DBusMenu {
     fn event(&self, id: i32, event_id: &str, data: &OwnedValue, timestamp: u32)
         -> zbus::Result<()>;
 
+    fn about_to_show(&self, id: i32) -> zbus::Result<bool>;
+
     #[zbus(signal)]
     fn layout_updated(&self, revision: u32, parent: i32) -> zbus::Result<()>;
 }