dzl-column-layout: Improve columns estimation
Improve the n_columns calculation in dzl_column_layout_layout() to avoid overshooting the required width by one count of column_spacing.
Fixes: gnome-builder#1037 (closed)
Improve the n_columns calculation in dzl_column_layout_layout() to avoid overshooting the required width by one count of column_spacing.
Fixes: gnome-builder#1037 (closed)