Removed the necessity to append the detected object index (related to the frame) to the effect JSON - which makes the JSON smaller and the performance better.
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)
Added "has_tracked_object" property on EffectBase so that the AddEffect function checks for this property instead of the Tracker effect name - this way it's possible to support other effects that have tracked objects but different names.
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.
Fixed bug that caused the clip to not attach correctly if the tracked object's parent clip wasn't in the position 0 of the timeline.
Fixed bug that caused the wrong scale adjustment when an emoji or picture was attached to a tracked object.
Added new base class TrackedObjectBase, changed class KeyframeBBox to TrackedObjectBBox and changes it's inheritance from KeyframeBase to TrackedObjectBase.
The Clip's Keyframes are no longer modified when it's attached to an object, and the bounding-box properties are requested by the Timeline's frame, not the Clip's frame.
