ext/alsa/: Make the xrun code timestamp and offset the buffers correctly. moved the clock to the base class, use alsa...

Original commit message from CVS:
* ext/alsa/gstalsa.c: (gst_alsa_init), (gst_alsa_dispose),
(gst_alsa_get_time), (gst_alsa_xrun_recovery):
* ext/alsa/gstalsa.h:
* ext/alsa/gstalsaclock.c: (gst_alsa_clock_get_type):
* ext/alsa/gstalsasrc.c: (gst_alsa_src_init), (gst_alsa_src_loop),
(gst_alsa_src_change_state):
* ext/alsa/gstalsasrc.h:
Make the xrun code timestamp and offset the buffers correctly.
moved the clock to the base class, use alsa methods to get time.
Do correct timestamping on outgoing buffers.