fix(wlr/taskbar): protocol error when reconnecting outputs

Destroy request is not specified for foreign toplevel manager and it
does not prevent the compositor from sending more events.
Libwayland would ignore events to a destroyed objects, but that could
indirectly cause a gap in the sequence of new object ids and trigger
error condition in the library.

With this commit waybar sends a `stop` request to notify the compositor
about the destruction of a toplevel manager. That fixes abnormal
termination of the bar with following errors:
```
(waybar:11791): Gdk-DEBUG: 20:04:19.778: not a valid new object id (4278190088), message toplevel(n)

Gdk-Message: 20:04:19.778: Error reading events from display: Invalid argument
```

src/modules/wlr/taskbar.cpp

@@ -637,8 +637,20 @@ Taskbar::Taskbar(const std::string &id, const waybar::Bar &bar, const Json::Valu
 Taskbar::~Taskbar()
 {
     if (manager_) {
-        zwlr_foreign_toplevel_manager_v1_destroy(manager_);
-        manager_ = nullptr;
+        struct wl_display *display = Client::inst()->wl_display;
+        /*
+         * Send `stop` request and wait for one roundtrip.
+         * This is not quite correct as the protocol encourages us to wait for the .finished event,
+         * but it should work with wlroots foreign toplevel manager implementation.
+         */
+        zwlr_foreign_toplevel_manager_v1_stop(manager_);
+        wl_display_roundtrip(display);
+
+        if (manager_) {
+            spdlog::warn("Foreign toplevel manager destroyed before .finished event");
+            zwlr_foreign_toplevel_manager_v1_destroy(manager_);
+            manager_ = nullptr;
+        }
     }
 }