yuzu: create linux group in general settings

- Create files dedicated to starting and stopping gamemode functions
  - Use them in yuzu and yuzu_cmd modules

src/common/linux/gamemode.h

@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: Copyright 2023 yuzu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+namespace Common::Linux {
+
+/**
+ * Start the (Feral Interactive) Linux gamemode if it is installed and it is activated
+ */
+void StartGamemode();
+
+/**
+ * Stop the (Feral Interactive) Linux gamemode if it is installed and it is activated
+ */
+void StopGamemode();
+
+/**
+ * Start or stop the (Feral Interactive) Linux gamemode if it is installed and it is activated
+ * @param state The new state the gamemode should have
+ */
+void SetGamemodeState(bool state);
+
+} // namespace Common::Linux