Add util method for scaling a clocktime by a fraction. Useful implementation is left as an exercise for the reader.

Original commit message from CVS:
* gst/gstutils.c: (gst_util_clocktime_scale):
* gst/gstutils.h:
* docs/gst/gstreamer-sections.txt:
Add util method for scaling a clocktime by a fraction. Useful
implementation is left as an exercise for the reader.