appDisplay: Disable scrolling during animations

appDisplay: Disable scrolling during animations

Currently scroll events during the swarm animation will make the
grid appear immediately in addition to the animating clones, and
there'll be a mismatch with the icon at the target position. This
badly breaks the illusion of launchers emerging from the dash and
positioning themselves in a grid - as scrolling icons "mid-air"
before they form a paginated grid doesn't make much sense anyway,
fix this issue by ignoring scroll events for the duration of the
animation.

https://bugzilla.gnome.org/show_bug.cgi?id=745574