Volume slider that adjusts how loud games are on a global level (#3240)

* Update config.cpp

* Update config.h

* Update sdl_audio.cpp

* Update settings_dialog.cpp

* Update settings_dialog.h

* Update settings_dialog.ui

* Update gui_settings.h

* Update audioout.cpp

* Update audioout.h

* Update settings_dialog.cpp

* remove leftover settings_dialog.ui

* Update settings_dialog.ui

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Co-authored-by: georgemoralis <giorgosmrls@gmail.com>

src/common/config.h

@@ -19,6 +19,8 @@ enum HideCursorState : int { Never, Idle, Always };
 void load(const std::filesystem::path& path);
 void save(const std::filesystem::path& path);
 
+int getVolumeSlider();
+void setVolumeSlider(int volumeValue);
 std::string getTrophyKey();
 void setTrophyKey(std::string key);
 bool getIsFullscreen();