ext/pango/gstclockoverlay.*: API: Add ability to specify format for date/time display by adding a "time-format" prope...

Original commit message from CVS:
Patch by: Pavel Zeldin <pzeldin at gmail dot com>
* ext/pango/gstclockoverlay.c: (gst_clock_overlay_render_time),
(gst_clock_overlay_class_init), (gst_clock_overlay_finalize),
(gst_clock_overlay_init), (gst_clock_overlay_set_property),
(gst_clock_overlay_get_property):
* ext/pango/gstclockoverlay.h:
API: Add ability to specify format for date/time display by
adding a "time-format" property.
Fixes bug #554879.