dash: Set scale and opacity on parent actor

dash: Set scale and opacity on parent actor

DashItemContainer currently animates the scale and opacity
of its child when zooming in. This is visible when adding
a new favorite item to the dash; the items will zoom in from
the center.

After the previous commit, however, the zoom animation got
slightly broken, and looked like the icon was coming from
the bottom instead of the center.

Fix that by setting the scale and opacity of DashItemContainer
itself, instead of its child. Remove the unused code after that
too.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/153