WindowServer: Add WindowServer::async_set_window_parent_from_client()

This allows a client to set the parent for one of its local windows
where the parent window is in another client.

Userland/Services/WindowServer/ClientConnection.cpp

@@ -1115,4 +1115,21 @@ void ClientConnection::set_flash_flush(bool enabled)
     Compositor::the().set_flash_flush(enabled);
 }
 
+void ClientConnection::set_window_parent_from_client(i32 client_id, i32 parent_id, i32 child_id)
+{
+    auto child_window = window_from_id(child_id);
+    if (!child_window)
+        did_misbehave("SetWindowParentFromClient: Bad child window ID");
+
+    auto client_connection = from_client_id(client_id);
+    if (!client_connection)
+        did_misbehave("SetWindowParentFromClient: Bad client ID");
+
+    auto parent_window = client_connection->window_from_id(parent_id);
+    if (!parent_window)
+        did_misbehave("SetWindowParentFromClient: Bad parent window ID");
+
+    child_window->set_parent_window(*parent_window);
+}
+
 }

Userland/Services/WindowServer/ClientConnection.h

@@ -164,6 +164,7 @@ private:
     virtual Messages::WindowServer::IsWindowModifiedResponse is_window_modified(i32) override;
     virtual Messages::WindowServer::GetDesktopDisplayScaleResponse get_desktop_display_scale(u32) override;
     virtual void set_flash_flush(bool) override;
+    virtual void set_window_parent_from_client(i32, i32, i32) override;
 
     Window* window_from_id(i32 window_id);
 

Userland/Services/WindowServer/WindowServer.ipc

@@ -147,4 +147,6 @@ endpoint WindowServer
     get_desktop_display_scale(u32 screen_index) => (int desktop_display_scale)
 
     set_flash_flush(bool enabled) =|
+
+    set_window_parent_from_client(i32 client_id, i32 parent_id, i32 child_id) =|
 }