WindowServer: Add API to set/get screen layouts

This sets the stage so that DisplaySettings can configure the screen
layout and set various screen resolutions in one go. It also allows
for an easy "atomic" revert of the previous settings.

Userland/Services/WindowServer/ScreenLayout.h

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <AK/Vector.h>
+#include <LibCore/ConfigFile.h>
+#include <LibGfx/Rect.h>
+#include <LibGfx/Size.h>
+#include <LibIPC/Forward.h>
+
+namespace WindowServer {
+
+class ScreenLayout {
+public:
+    struct Screen {
+        String device;
+        Gfx::IntPoint location;
+        Gfx::IntSize resolution;
+        int scale_factor;
+
+        Gfx::IntRect virtual_rect() const
+        {
+            return { location, { resolution.width() / scale_factor, resolution.height() / scale_factor } };
+        }
+
+        auto operator<=>(const Screen&) const = default;
+    };
+
+    Vector<Screen> screens;
+    unsigned main_screen_index { 0 };
+
+    bool is_valid(String* error_msg = nullptr) const;
+    void normalize();
+    bool load_config(const Core::ConfigFile& config_file, String* error_msg = nullptr);
+    bool save_config(Core::ConfigFile& config_file, bool sync = true) const;
+
+    // TODO: spaceship operator
+    bool operator!=(const ScreenLayout& other) const;
+    bool operator==(const ScreenLayout& other) const
+    {
+        return !(*this != other);
+    }
+};
+
+}
+
+namespace IPC {
+
+bool encode(Encoder&, const WindowServer::ScreenLayout::Screen&);
+bool decode(Decoder&, WindowServer::ScreenLayout::Screen&);
+bool encode(Encoder&, const WindowServer::ScreenLayout&);
+bool decode(Decoder&, WindowServer::ScreenLayout&);
+
+}