WindowServer: Make window stealing related functions synchronous

This allows us to make sure that those operations are performed
in the right order. Affected functions are:
 - add_window_stealing_for_client
 - set_window_parent_from_client
 - remove_window_stealing_for_client
 - remove_window_stealing

Author: LucasChollet

Userland/Services/WindowServer/WindowServer.ipc

@@ -176,9 +176,9 @@ endpoint WindowServer
 
     set_flash_flush(bool enabled) =|
 
-    set_window_parent_from_client(i32 client_id, i32 parent_id, i32 child_id) =|
+    set_window_parent_from_client(i32 client_id, i32 parent_id, i32 child_id) => ()
     get_window_rect_from_client(i32 client_id, i32 window_id) => (Gfx::IntRect rect)
-    add_window_stealing_for_client(i32 client_id, i32 window_id) =|
-    remove_window_stealing_for_client(i32 client_id, i32 window_id) =|
-    remove_window_stealing(i32 window_id) =|
+    add_window_stealing_for_client(i32 client_id, i32 window_id) => ()
+    remove_window_stealing_for_client(i32 client_id, i32 window_id) => ()
+    remove_window_stealing(i32 window_id) => ()
 }