Revert "derive clips, tracks, and sequences from the same base"

This reverts commit 1b9a963eeb.

timeline/sequence.h

@@ -29,9 +29,8 @@
 #include "selection.h"
 #include "tracklist.h"
 #include "ghost.h"
-#include "timelineobject.h"
 
-class Sequence : public TimelineObject {
+class Sequence : public QObject {
   Q_OBJECT
 public:
   Sequence();
@@ -46,12 +45,6 @@ public:
   int audio_frequency;
   int audio_layout;
 
-  virtual int SequenceWidth() override;
-  virtual int SequenceHeight() override;
-  virtual double SequenceFrameRate() override;
-
-  virtual long SequencePlayhead() override;
-
   long GetEndFrame();
   QVector<Clip*> GetAllClips();
   TrackList* GetTrackList(olive::TrackType type);