appDisplay: Animate DnD app icons at the position they were dropped

Indicate whether dropping an app icon was successful or not by using the
newly added `animateLaunchAtPos()` API of AppIcon which starts a zoom
out animation of the icon at the position the drop happened.

To get the position of the drag actor, we have to forward the arguments
passed to `acceptDrop()` and `handleDragOver()` to the internal drag
handlers of the WorkspaceThumbnails. We can use this position directly
without transforming it to stage coordinates because the actor is a
child of `Main.uiGroup` and the animation actor will also be a child of
this container.

GNOME/gnome-shell!121
6 jobs for !121 with dash-fixes in 6 minutes and 56 seconds (queued for 6 seconds)
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Status Job ID Name Coverage
  Review
passed #436771
check_commit_log

00:00:32

passed #436773
eslint

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passed #436772
js_check

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  Build
passed #436774
build

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  Test
passed #436775
test

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passed #436776
test-pot

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