make GstElementDetails const

make GstElementDetails const

Original commit message from CVS:
* gst/gstbin.c:
* gst/gstpipeline.c:
* plugins/elements/gstcapsfilter.c:
* plugins/elements/gstfakesink.c:
* plugins/elements/gstfakesrc.c:
* plugins/elements/gstfdsink.c:
* plugins/elements/gstfdsrc.c:
* plugins/elements/gstfilesink.c:
* plugins/elements/gstfilesrc.c:
* plugins/elements/gstidentity.c:
* plugins/elements/gstqueue.c:
* plugins/elements/gsttee.c:
* plugins/elements/gsttypefindelement.c:
(gst_type_find_element_handle_event):
make GstElementDetails const