status-area: Use ItemIsMenu correctly

The comment that was previously here assumed that `ItemIsMenu` should be
defined for any status item that has a menu, but the FreeDesktop spec
rather defines it as an item that "only supports the context menu", so
we should open the menu instead of trying to call `Activate`.

This change won't affect behavior if `ItemIsMenu` is true, but `Activate`
errors, except by opening the menu without having to wait for that
errror first. It will fix the left click to open menu behavior if any
client defines that method to not error, but does set `ItemIsMenu`.

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

@@ -543,6 +543,9 @@ fn activate(
     item: &StatusNotifierItem,
     activation_token: Option<String>,
 ) -> Task<cosmic::Action<Msg>> {
+    if item.is_menu() {
+        return Task::done(cosmic::action::app(Msg::TogglePopup(id)));
+    }
     let item_proxy = item.item_proxy().clone();
     Task::future(async move {
         if let Some(t) = activation_token {