WindowServer+LibGUI: Make menubar allocation asynchronous

Same as with menu allocation, move menubar ID management to the client
side, removing more IPC stalls during application startup.

Userland/Libraries/LibGUI/Menubar.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/Badge.h>
+#include <AK/IDAllocator.h>
 #include <LibGUI/Menu.h>
 #include <LibGUI/MenuItem.h>
 #include <LibGUI/Menubar.h>
@@ -12,6 +13,8 @@
 
 namespace GUI {
 
+static IDAllocator s_menubar_id_allocator;
+
 Menubar::Menubar()
 {
 }
@@ -30,7 +33,9 @@ Menu& Menubar::add_menu(String name)
 
 int Menubar::realize_menubar()
 {
-    return WindowServerConnection::the().create_menubar();
+    auto menubar_id = s_menubar_id_allocator.allocate();
+    WindowServerConnection::the().async_create_menubar(menubar_id);
+    return menubar_id;
 }
 
 void Menubar::unrealize_menubar()

Userland/Services/WindowServer/ClientConnection.cpp

@@ -81,12 +81,10 @@ void ClientConnection::notify_about_new_screen_rect(Gfx::IntRect const& rect)
     async_screen_rect_changed(rect);
 }
 
-Messages::WindowServer::CreateMenubarResponse ClientConnection::create_menubar()
+void ClientConnection::create_menubar(i32 menubar_id)
 {
-    int menubar_id = m_next_menubar_id++;
     auto menubar = Menubar::create(*this, menubar_id);
     m_menubars.set(menubar_id, move(menubar));
-    return menubar_id;
 }
 
 void ClientConnection::destroy_menubar(i32 menubar_id)

Userland/Services/WindowServer/ClientConnection.h

@@ -89,7 +89,7 @@ private:
     void destroy_window(Window&, Vector<i32>& destroyed_window_ids);
 
     virtual Messages::WindowServer::GreetResponse greet() override;
-    virtual Messages::WindowServer::CreateMenubarResponse create_menubar() override;
+    virtual void create_menubar(i32) override;
     virtual void destroy_menubar(i32) override;
     virtual void create_menu(i32, String const&) override;
     virtual void destroy_menu(i32) override;
@@ -160,7 +160,6 @@ private:
 
     RefPtr<Core::Timer> m_ping_timer;
 
-    int m_next_menubar_id { 10000 };
     int m_next_window_id { 1982 };
 
     bool m_has_display_link { false };

Userland/Services/WindowServer/WindowServer.ipc

@@ -2,7 +2,7 @@ endpoint WindowServer
 {
     greet() => (Gfx::IntRect screen_rect, Core::AnonymousBuffer theme_buffer)
 
-    create_menubar() => (i32 menubar_id)
+    create_menubar(i32 menubar_id) =|
     destroy_menubar(i32 menubar_id) =|
 
     create_menu(i32 menu_id, [UTF8] String menu_title) =|