keyframes: only update active frames even from curve and param editor

When adjusting a slider, keyframe time, or curve value, it is undesirable to
re-cache the entire affected area while the user is still dragging UI objects.
Since the video is unlikely to be playing, the priority must go to the
currently active frame so the user gets visual feedback on the rendered image
as soon as possible. This was implemented in some areas, but this commit should
have that functionality in all areas.

app/widget/timelinewidget/timelinescaledobject.cpp

@@ -8,12 +8,12 @@ TimelineScaledObject::TimelineScaledObject() :
 
 }
 
-const rational &TimelineScaledObject::timebase()
+const rational &TimelineScaledObject::timebase() const
 {
   return timebase_;
 }
 
-const double &TimelineScaledObject::timebase_dbl()
+const double &TimelineScaledObject::timebase_dbl() const
 {
   return timebase_dbl_;
 }