DisplaySettings: Add new Desktop tab with virtual desktop settings

This allows the user to configure the virtual desktop setup as desired.

Userland/Applications/DisplaySettings/DesktopSettingsWidget.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibCore/Timer.h>
+#include <LibGUI/SpinBox.h>
+
+namespace DisplaySettings {
+
+class DesktopSettingsWidget : public GUI::Widget {
+    C_OBJECT(DesktopSettingsWidget);
+
+public:
+    virtual ~DesktopSettingsWidget() override;
+
+    void apply_settings();
+
+private:
+    DesktopSettingsWidget();
+
+    void create_frame();
+    void load_current_settings();
+
+    RefPtr<GUI::SpinBox> m_virtual_desktop_rows_spinbox;
+    RefPtr<GUI::SpinBox> m_virtual_desktop_columns_spinbox;
+};
+
+}