- Make the return GList of gst_element_get_pad_list as const

- Make the return GList of gst_element_get_pad_list as const

Original commit message from CVS:
- Make the return GList of gst_element_get_pad_list as const
- Pad scheduler refactoring
- Remove gst_pad_set/unset_scheduler as it's not needed
- Reimplement gst_pad_get_scheduler using the parent scheduler
- Remove gst_pad_peek as it cannot work reliably
- Remove the pad bufpen, replace with scheduler private gpointer
- Make queue use the new _get_scheduler implementation
- Remove _pad_unset_scheduler from GstScheduler
- Remove LOOP_SEEK mode from aggregator
- Other fixes for API changes.