gst/: Added start/stop methods to transform base class so subclasses don't need to deal with state changes even.

Original commit message from CVS:
* gst/base/gstbasetransform.c: (gst_base_transform_base_init),
(gst_base_transform_class_init), (gst_base_transform_init),
(gst_base_transform_setcaps), (gst_base_transform_handle_buffer),
(gst_base_transform_get_property),
(gst_base_transform_sink_activate),
(gst_base_transform_src_activate),
(gst_base_transform_change_state):
* gst/base/gstbasetransform.h:
* gst/elements/gstidentity.c: (gst_identity_class_init),
(gst_identity_event), (gst_identity_check_perfect),
(gst_identity_transform), (gst_identity_start),
(gst_identity_stop):
Added start/stop methods to transform base class so subclasses
don't need to deal with state changes even.