First eliminate the variable WELL_ITEM_HSPACING since it was 0
and thus effectively was not used.
Add a new variable WELL_ITEM_MIN_HSPACING which is the minimum
space between grid items we allow. When computing layout, allow
for a number of columns less than 4 by using the minimum item
size.
https://bugzilla.gnome.org/show_bug.cgi?id=595023
Based on a patch from Colin Walters.