appDisplay: Use ::paint handler to set up spring animation

appDisplay: Use ::paint handler to set up spring animation

The animation needs the icons' final positions, so we currently defer
it to a ::notify::allocation handler; however as starting the animation
during an allocation cycle would trigger a Clutter warning, it is
further deferred to a MetaLater. While this usually works, it is possible
that the allocation is already valid when we connect the signal, in which
case the animation is triggered at a later unexpected time. Switch to
a more robust ::paint handler instead, which also allows us to get rid
of the double-delay.

https://bugzilla.gnome.org/show_bug.cgi?id=736148