- some pad.h reorg, better grouping of function

- some pad.h reorg, better grouping of function

Original commit message from CVS:
- some pad.h reorg, better grouping of function
- added methods and default implementations to get supported formats,
seek methods/flags and query types.
- implemented pad activation/disabling
- start negotiation in the READY->PAUSED state
- added GST_PAD_IS_USABLE (better name?) to check if a pad can be used
for data transport (check if connected and peerpad is active)
- added query for segment end