[3.0] VLCMediaPlayer: update setCurrentVideoSubTitleIndex to disable subtitles

Passing -1 now disables the subtitles matching the documentation. Fixes #585 (closed)

This backports the patch proposed in !130 (closed) for the 3.0 branch while for master, the MR is superseded by !187 (merged).

changed milestone to %3.5.0

added Maintenance label

assigned to @fkuehne

mentioned in merge request !130 (closed)

mentioned in issue #585 (closed)

closed