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/components/app.rs

@@ -150,6 +150,8 @@ impl cosmic::Application for App {
                     None
                 };
                 if self.open_menu.is_some() {
+                    self.menus[&id].about_to_show();
+
                     let mut cmds = Vec::new();
                     if let Some(id) = self.popup.take() {
                         cmds.push(destroy_popup(id));

cosmic-applet-status-area/src/components/status_menu.rs

@@ -83,6 +83,13 @@ impl State {
     pub fn subscription(&self) -> iced::Subscription<Msg> {
         self.item.layout_subscription().map(Msg::Layout)
     }
+
+    pub fn about_to_show(&self) {
+        let menu_proxy = self.item.menu_proxy().clone();
+        tokio::spawn(async move {
+            let _ = menu_proxy.about_to_show(0).await;
+        });
+    }
 }
 
 fn layout_view(layout: &Layout, expanded: Option<i32>) -> cosmic::Element<Msg> {

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<()>;
 }