Settings menu: display settings, language codes, style guide window

Display tab: window mode (windowed/maximized/borderless/exclusive), monitor, window
size per monitor (centred, disabled in fullscreen), V-Sync, frame rate cap, reset;
applied and saved at once through the core DisplaySettingsService. Language is read
through LanguageCode (en-en/en_US = English). The window follows the style guide
shell: title and tabs in the header, close on the right, footer hint and per-tab reset.
The main menu hides every session panel of UiHost (pause menu stayed above it).
New suite display_settings (+ --shots); collection_edit leaves the game through the
pause menu and checks it is closed.

test_suites/display_settings/src/DisplaySettingsTestInitializer.cs

@@ -0,0 +1,33 @@
+using System.Linq;
+using Cthangover.Core.Mods;
+using Cthangover.Core.Utils;
+using Godot;
+
+namespace Mods.DisplaySettingsTest
+{
+    /// <summary>Queues <see cref="DisplaySettingsTestDriverNode"/> only for <c>--itest=display_settings</c>; <c>--shots=&lt;dir&gt;</c> makes it save screenshots of the menu and every settings tab.</summary>
+    public class DisplaySettingsTestInitializer : IModInitializer
+    {
+        private static bool queued;
+
+        public void OnModLoaded(string modId) { }
+
+        public void OnModResourcesReady()
+        {
+            var args = OS.GetCmdlineUserArgs();
+            if (queued || !args.Contains("--itest=display_settings"))
+                return;
+
+            string shots = null;
+            foreach (var arg in args)
+            {
+                if (arg.StartsWith("--shots="))
+                    shots = arg.Substring("--shots=".Length);
+            }
+
+            queued = true;
+            GameLogger.Log("TEST", "DisplaySettingsTest: activating");
+            (Godot.Engine.GetMainLoop() as SceneTree)?.Root?.CallDeferred("add_child", new DisplaySettingsTestDriverNode(shots));
+        }
+    }
+}