Save media time every 20sec to prevent progress loss

Fixes #2410 (closed)

Note, this is working per se, but any media save (including setting a time) will trigger the CallBackDelegate::onMediaModified callback, forcing the refresh of any loaded viewmodel.

It can result in hundreds of SQL queries during a playback that is not wanted at all.

To prevent that, we should stop the refresh from happening and postpone it when the calling fragment is shown again.