ges: Fix the TimelineObject::effect-added signal emission timing
ges: Fix the TimelineObject::effect-added signal emission timing

We were emitting it before it gets added to the track_object list,
so the list we were getting with get_top_effects was containing
a TrackObject that wasen't a TrackEffect

+ A bit of refactoring

MathieuDuponchelle/PitiviGes,MathieuDuponchelle/PitiviGes,MathieuDuponchelle/PitiviGes,MathieuDuponchelle/PitiviGes