workspacesView: Avoid an unnecessary animation

workspacesView: Avoid an unnecessary animation

Currently we animate scrolling to the active workspace both when
the number of workspaces changed and after changing the active
workspace. So in case we don't actually change workspace, this
results in an unnecessary animation that may even have unwanted
side effects: when done during the overview transition (e.g. in
the case of opening and activating a window on an empty workspace),
non-active workspaces become visible during the transition.
To fix, don't scroll to the active workspace when the number of
workspaces changes and rely on the 'switch-workspace' signal being
emitted as necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=682002