Subtitles: work on temp files until we save the project so that each change to the subtitles is not instantly saved

Fix undo import subtitle file, and improve subtitle group operations (only reload subtitle file once)

Fix subtitle model used start time as index - caused issues if a subtitle if moved after another one. Add start/end position control in subtitle widget

Fix crash loading some ass subtitles, add basic widget for subtitle edit otherwise the feature was not much usable

Related to #666