implemented core UI caching feedback functionality

Not a perfect implementation yet, but this shows UI feedback on what frames are
cached and which ones aren't.

app/widget/timelinewidget/view/timelineviewbase.cpp

@@ -14,6 +14,7 @@ TimelineViewBase::TimelineViewBase(QWidget *parent) :
   playhead_(0),
   playhead_scene_left_(-1),
   playhead_scene_right_(-1),
+  dragging_hand_(false),
   limit_y_axis_(false)
 {
   setScene(&scene_);