search_gui_show_search(): reuse already computed "column" value.
[GTK2] Added tri-state column sorting logic in the "Downloads" pane.
[GTK2] Added tri-state column sorting logic in the "Uploads" pane.
Regroup sorting context variables (column and order) in common structure.
[GTK2] Factorized tri-state column sorting logic.
[GTK2] Added tri-state column sorting to the "search list" pannel.
Ensure we never insert any "unusable host" in the push-proxy set.