Do not call the visible_func() excesively in HildonLiveSearch
Do not call the visible_func() excesively in HildonLiveSearch

During the mapping of the selection, visible_func() was heavily used
to find out whether a model row is visible or not. Instead, this could
be done by checking whether filter_path is valid, then saving many
calls to the visible_func().

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