gst/sine/gstsinesrc.*: add num-buffers and timestamp-offset properties

gst/sine/gstsinesrc.*: add num-buffers and timestamp-offset properties

Original commit message from CVS:

* gst/sine/gstsinesrc.c: (gst_sinesrc_class_init),
(gst_sinesrc_init), (gst_sinesrc_create),
(gst_sinesrc_set_property), (gst_sinesrc_get_property),
(gst_sinesrc_start):
* gst/sine/gstsinesrc.h:
add num-buffers and timestamp-offset properties
* gst/videotestsrc/gstvideotestsrc.c:
(gst_videotestsrc_class_init), (gst_videotestsrc_set_property),
(gst_videotestsrc_get_property):
add timestamp-offset property