Search: Don't use IconGrid width for maxDisplayedResults

Currently to know how many results we could show for GridResults
we use the width of the bin containing those results. Since it's
expanding it shouldn't be a problem. But it becomes a problem when
no results are displayed, thus the container becomes hidden and
it losts its allocation.
In the next introduction of terms in search we call again
maxDisplayedResults but it doesn't have allocation yet, and therefore no
results are displayed (currently a bug on IconGrid makes the min size =
one icon, so actually we show one and only one icon in this case).

To solve that use the parent container which contains the search results
of all providers or the text label with not displayed results, so it
always have the real available width to calculate maxDisplayedResults.

Thanks Alban Browaeys for the debugging footwork.

https://bugzilla.gnome.org/show_bug.cgi?id=732416