gst/gstbin.*: Removing a clock provider from a bin, triggers a clock lost message so that a new clock will be selected.

Original commit message from CVS:
* gst/gstbin.c: (gst_bin_provide_clock_func), (gst_bin_add_func),
(gst_bin_remove_func), (bin_bus_handler):
* gst/gstbin.h:
Removing a clock provider from a bin, triggers a clock lost message
so that a new clock will be selected.
Adding a clock to a bin triggers a clock provider message.
Make sure we reselect a clock when we received a clock lost message.
Keep a reference to the element that provided the clock.