dash: Don't underallocate show-apps button

When the dash contains more icons than fit at the minimum icon size,
icons are cut off at the end. This means that the show-apps button
will be the first to disappear, which is problematic given it's the
sole access point for other applications (for those that refuse to
use search at least).
Fix by using a dedicated widget for the dash actor, so that in case
of underallocation only icons above the show-apps button end up being
cut off.

https://bugzilla.gnome.org/show_bug.cgi?id=683340