Refactoring of WListbox widget.

Use GQueue instead of GList to store listbox entries.

g_list_append() function is slow because it uses g_list_last()
internally to traverse from the beginning to the end of a list, so
forming a list of results has O(n*n) complexity instead of O(n).
GQueue contains pointers to head and tail of list and list length.
So in this case we don't need seach end of list every time when we
want append listbox entry to the listbox.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>