libvlc: add new API to set AB loop explicitly

Adds:

• libvlc_media_player_set_abloop_time
• libvlc_media_player_set_abloop_position
• libvlc_media_player_reset_abloop

These new methods allow to set a specific start and end time or position to setup an AB loop.

This is in contrast to libvlc_media_player_set_abloop that sets the AB loop bounds based on the current playback time.

There must be an active input before setting the AB loop.