When we have more thumbnails than can fit in the vertical space, scale
them down. This is implemented by using a generic container so we can
compute positions and sizes on the fly and do the appropriate
width-for-height behavior.
The usage of clutter constraints to position the indicator is droppped
since it less complicated to just position the indicator in the right
place ourselves.
https://bugzilla.gnome.org/show_bug.cgi?id=641879