overview: Move dash to a separate layer

overview: Move dash to a separate layer

We generally want view content centered, in particular where the
view itself is symmetrical. So move the dash to a separate layer
and use a placeholder to account for its size when showing the
window picker, which is the only view where it doesn't make sense
to center the content.

https://bugzilla.gnome.org/show_bug.cgi?id=694261