LibGUI: Add a simple "recently open files" feature

This feature allows any application to easily install an automatically
updating list of recently open files in a GUI::Menu.

There are three main pieces to this mechanism:

- GUI::Application::set_config_domain(domain): This must be called
  before using the recent files feature. It informs the Application
  object about which config domain to find the relevant RecentFiles
  list under.

- GUI::Menu::add_recently_open_files(callback): This inserts the list
  in a menu. A callback must be provided to handle actually opening
  the recent file in some application-specific way.

- GUI::Application::set_most_recently_open_file(path): This updates
  the list of recently open files, both in the configuration files,
  and in the GUI menu.

Userland/Libraries/LibGUI/Application.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/NeverDestroyed.h>
+#include <LibConfig/Client.h>
 #include <LibCore/EventLoop.h>
 #include <LibGUI/Action.h>
 #include <LibGUI/Application.h>
@@ -324,4 +325,74 @@ void Application::event(Core::Event& event)
     Object::event(event);
 }
 
+void Application::set_config_domain(String config_domain)
+{
+    m_config_domain = move(config_domain);
+}
+
+void Application::register_recent_file_actions(Badge<GUI::Menu>, Vector<NonnullRefPtr<GUI::Action>> actions)
+{
+    m_recent_file_actions = move(actions);
+    update_recent_file_actions();
+}
+
+void Application::update_recent_file_actions()
+{
+    VERIFY(!m_config_domain.is_empty());
+
+    size_t number_of_recently_open_files = 0;
+    auto update_action = [&](size_t index) {
+        auto& action = m_recent_file_actions[index];
+        char buffer = static_cast<char>('0' + index);
+        auto key = StringView(&buffer, 1);
+        auto path = Config::read_string(m_config_domain, "RecentFiles"sv, key);
+
+        if (path.is_empty()) {
+            action->set_visible(false);
+            action->set_enabled(false);
+        } else {
+            action->set_visible(true);
+            action->set_enabled(true);
+            action->set_text(path);
+            ++number_of_recently_open_files;
+        }
+    };
+    for (size_t i = 0; i < max_recently_open_files(); ++i)
+        update_action(i);
+
+    // Hide or show the "(No recently open files)" placeholder.
+    m_recent_file_actions.last()->set_visible(number_of_recently_open_files == 0);
+}
+
+void Application::set_most_recently_open_file(String new_path)
+{
+    Vector<DeprecatedString> new_recent_files_list;
+
+    for (size_t i = 0; i < max_recently_open_files(); ++i) {
+        static_assert(max_recently_open_files() < 10);
+        char buffer = static_cast<char>('0' + i);
+        auto key = StringView(&buffer, 1);
+        new_recent_files_list.append(Config::read_string(m_config_domain, "RecentFiles"sv, key));
+    }
+
+    new_recent_files_list.remove_all_matching([&](auto& existing_path) {
+        return existing_path.view() == new_path;
+    });
+
+    new_recent_files_list.prepend(new_path.to_deprecated_string());
+
+    for (size_t i = 0; i < max_recently_open_files(); ++i) {
+        auto& path = new_recent_files_list[i];
+        char buffer = static_cast<char>('0' + i);
+        auto key = StringView(&buffer, 1);
+        Config::write_string(
+            m_config_domain,
+            "RecentFiles"sv,
+            key,
+            path);
+    }
+
+    update_recent_file_actions();
+}
+
 }