Improve terminology in the code

• TimelineWidget ⇒ TimePane
• TimelineScale ⇒ TimePaneScale
• TimelineLine ⇒ Timeline
• TimelineLineLabel ⇒ TimelineLabel
• TimelineLineDialog⇒ TimelineDialog
• AnnotationGroup ⇒ Event
• AnnotationGroupDialog ⇒ EventDialog
• Indicator ⇒ Cursor
• Player ⇒ Video