implemented base UI keyframe functionality

This commit implements much of the base functionality for creating and
destroying keyframes in the UI. Not complete yet, but getting there.

app/widget/timelinewidget/view/timelineviewbase.cpp

@@ -32,6 +32,8 @@ void TimelineViewBase::SetScale(const double &scale)
   viewport()->update();
 
   end_item_->SetScale(scale_);
+
+  ScaleChangedEvent(scale_);
 }
 
 void TimelineViewBase::SetTimebase(const rational &timebase)
@@ -146,3 +148,8 @@ void TimelineViewBase::resizeEvent(QResizeEvent *event)
 
   UpdateSceneRect();
 }
+
+void TimelineViewBase::ScaleChangedEvent(double scale)
+{
+  Q_UNUSED(scale)
+}