Use g_list_insert_sorted() instead of our custom function

Use g_list_insert_sorted() instead of our custom function

The clutter_timeout_pool_insert_sorted() function caused an inversion of
the timeout sources in the pool; this led to a wrong behaviour in the
execution of the timeout functions. See bug 471.

This patch drops clutter_timeout_pool_insert_sorted() in favour of the
standard g_list_insert_sorted(), which produces identical behaviours with
and without the pool.

A new test, written by Rob Bradford, has been added to the regression test
suite in order to identify sorting issues with the timeout pools.