Use minimum/natural size semantics

Use minimum/natural size semantics

Rewrites gtk_tree_view_column_request_width() and
gtk_tree_view_size_allocate_columns() to respect the minimum and natural
sizes that are already being returned by
gtk_cell_area_context_get_preferred_width().

The convoluted logic explained (not!) by this comment has been removed:
“Only update the expand value if the width of the widget has changed, or
the number of expand columns has changed, or if there are no expand
columns, or if we didn't have an size-allocation yet after the last
validated node.”  This logic seems to have been a workaround for the
“jumping” behavior fixed in 16195ad and is no longer necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=691751