Do not set the internal pad as a parent anymore so we can avoid hierarchy linking errors when the ghostpad has no par...

Original commit message from CVS:
* docs/design/part-gstghostpad.txt:
* gst/gstghostpad.c: (gst_ghost_pad_class_init),
(gst_ghost_pad_internal_do_activate_push),
(gst_ghost_pad_internal_do_activate_pull),
(gst_ghost_pad_do_activate_push), (gst_ghost_pad_do_activate_pull),
(gst_ghost_pad_do_link), (gst_ghost_pad_dispose),
(gst_ghost_pad_new_full), (gst_ghost_pad_set_target):
Do not set the internal pad as a parent anymore so we can avoid
hierarchy linking errors when the ghostpad has no parent yet. This also
fixes failed activation because of unlinked internal pads, which in
turn fixes the impossible case where you have to activate a pad before
you can add it to a running element.
Also fix the docs.
* gst/gstpad.c: (pre_activate), (post_activate),
(gst_pad_set_active), (gst_pad_activate_pull),
(gst_pad_activate_push), (gst_pad_check_pull_range):
Add some more debug info.
Mark activation mode in pre_activate so that we don't try to activate in
endless loops. Fixes #385084.