Only show the tracked object's icon (on the mini-GUI to attach a clip to it) and transform handler if the object appears on the screen (i.e. it has data for the requested frame)
Fixed bug that made Openshot crash if two or more Tracker effects were added to the same video.
Fixed bug that made the clips to be incorrectly attached to tracked objects in the exported video.
Added new base class TrackedObjectBase, changed class KeyframeBBox to TrackedObjectBBox and changes it's inheritance from KeyframeBase to TrackedObjectBase.