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/yuzu/main.h

@@ -340,6 +340,7 @@ private:
     void SetupSigInterrupts();
     static void HandleSigInterrupt(int);
     void OnSigInterruptNotifierActivated();
+    void SetGamemodeEnabled(bool state);
 #endif
 
 private slots: