CLuaInstance: Add multiple script functions for mute icon & volume

- Functions: enableMuteIcon, isMuted, AudioMute, setVolume, getVolume
 - This allows to save volume/mute status at plugin start
   and restore it on exit.

 - Set Lua api version to 1.35

src/gui/lua/luainstance.h

@@ -31,7 +31,7 @@ extern "C" {
 #include "luainstance_helpers.h"
 
 #define LUA_API_VERSION_MAJOR 1
-#define LUA_API_VERSION_MINOR 34
+#define LUA_API_VERSION_MINOR 35
 
 void LuaInstRegisterFunctions(lua_State *L, bool fromThreads=false);