shell-helper: dim apps with new layer under panel when launcher is open

The animation now are kind of shonky. They should be more synced with
the launcher grid opening and closing but it's difficult to cancel
weston animations while they're still running. This is next to be
fixed.
shell-helper: update for new  weston_layer_entry struct

This is to reflect 412e6a59eb90d in weston. If you're using an older
weston you can safely revert this commit.