[HildonLiveSearch] Use selection map only when not in GTK_SELECTION_NONE
[HildonLiveSearch] Use selection map only when not in GTK_SELECTION_NONE

It is not necessary to maintain a selection mapping when the GtkTreeView's
selection mode is GTK_SELECTION_NONE, so don't waste time on it.

Fixes: NB#165632 (HildonLiveSearch wasting CPU in
       gtk_tree_model_filter_convert_child_path_to_path)

community-ssu/hildon,archlinuxarm-n900/libhildon,community-ssu/hildon,archlinuxarm-n900/libhildon,community-ssu/hildon,archlinuxarm-n900/libhildon