Add basic marker deleting and fix selection code

app/widget/timeruler/seekablewidget.h

@@ -53,7 +53,7 @@ public:
     return dragging_;
   }
 
-  virtual void DeleteSelected(){};
+  void DeleteSelected();
 
   QVector<TimelineMarker*> GetActiveTimelineMarkers();
 
@@ -68,6 +68,8 @@ public slots:
 
   void addMarker(TimelineMarker* marker);
 
+  void removeMarker(TimelineMarker* marker);
+
   void updateMarkerPositions();
 
   void SetMarkerColor(int c);