* src/nautilus-index-tabs.c: (nautilus_index_tabs_initialize),
(nautilus_index_tabs_size_request), (draw_one_tab),
(draw_or_hit_test_all_tabs), (tab_item_find_by_name),
(nautilus_index_tabs_add_view),
(nautilus_index_tabs_get_title_from_index),
(nautilus_index_tabs_remove_view),
(nautilus_index_tabs_prelight_tab),
(nautilus_index_tabs_select_tab), (nautilus_index_tabs_set_color),
(nautilus_index_tabs_receive_dropped_color),
(nautilus_index_tabs_set_title),
(nautilus_index_tabs_set_title_mode),
(nautilus_index_tabs_set_visible):

Some cleanup of index tabs.

Changed find_tab() to tab_item_find_by_name() which returns the
actual tab item instead of a list node.

Changed the GList append and remove operations to be more sane and
actually work in the latter case.

Print warnings on attempts to operate on non-existing tab items as
these indicate programming errors upstream.

Also add bulletproofing g_return_* macros.