don't emit Dialogic.timeline_ended until Timeline#clean has been called

[iscke]

this fixes a bug where starting a timeline in a callback attached to Dialogic.timeline_ended will lock, since Timeline#clean fires a frame after it's called (and after timeline_ended is emitted), which will then disconnect the signals on the new timeline.

(easy repro:

func _ready():
    Dialogic.start("whatever.dtl")
    Dialogic.timeline_ended.connect(Dialogic.start.bind("whatever.dtl")

causes the timeline to run once, and then to start a second time without input working.)

[Jowan-Spooner]

Looks good to me. Thanks!