Update Authors.txt to reflect the switch to Transifex.
Fix a possible dead lock when opening a file when one is already playing.

Easily reproducible with madVR and D3D Fullscreen.

Ticket #3865.
Fix: Update the chapter marks on the seekbar when the media length changes.

Mostly useful for MKV with multiple editions.

Fixes #3963.
Update translations from Transifex:

 - Greek.