searchDisplay: set a max width on the search results scrolled child

On large displays, we don't want the search results list to expand
across the whole screen; set a maximum width of 1000px.
Unfortunately, since in St max-width only affects size requisition, we
need a little custom layout manager to have it applied to the allocation
too.

https://bugzilla.gnome.org/show_bug.cgi?id=692453