WorkspaceThumbnails: animate the right workspace when creating by DND

To create a new workspace by dropping on the placeholder, we move all
the windows down one workspace and then wait for _checkWorkspaces() to
automatically create the empty one at the end.
This means that, from the implementation POV, the new workspace is not
the one created by DND but it's the last one, and this detail was exposed
in the UI because the animation was applied on that one.
Fix that by starting the animation manually from the DND code, and then
blocking the animation from happening when the new workspace is created
with a flag.

https://bugzilla.gnome.org/show_bug.cgi?id=685285