[Fix] Store subtitle settings only on user action

This is a follow-up on PR https://github.com/xbmc/xbmc/pull/4921.
Video settings are now stored **only** on user actions.

Thanks @Voyager1 for your help!
or else... CGUIDialogAudioSubtitleSettings::FrameMove() should not force the value to be taken from videoSettings.m_SubtitleOn, but rather g_application.m_pPlayer->GetSubtitleVisible(). The risk is only that visibility as set through the DVD menu never gets saved.