Update default.py

Playbacklock will be a boolean from now on, and track position manually instead of a seperate thread. The seperate thread and lock prevent the subtitles plugin from working/activating.
Update playback.py

- Changed playbacklock to a boolean and removed the thread responsible for tracking position. The thread and thread event block other processes from running.
- changed DB to db_dir in line 80.
Merge pull request #47 from Vlemert/patch_subtitles_resumeplayback

Patch subtitles resumeplayback