tweener: Save handlers on target and remove them on destroy

A tweener onComplete handler could be called when the target actor has already
been destroyed, and this could lead accessing to invalid properties of it.

So, wrap the handlers with lambda functions that we delete them when the object
state so that they are not called anymore after this point.

https://bugzilla.gnome.org/show_bug.cgi?id=791233
GNOME/gnome-shell!4
4 jobs for !4 with js-error-fixes in 6 minutes and 58 seconds (queued for 1 second)
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  Test
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passed #312748
test-pot

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