pad: Deprecate GST_FLOW_IS_FATAL() and GST_FLOW_IS_SUCCESS()

pad: Deprecate GST_FLOW_IS_FATAL() and GST_FLOW_IS_SUCCESS()

The problem with both macros is, that they suggest something that isn't true.
If GST_FLOW_IS_FATAL is true, there could still be a problem for many elements
and they should stop what they're currently doing and return that value
upstream (e.g. not-linked in a parser). If GST_FLOW_IS_SUCCESS is false, it
could still be that this is "ok" for the element (e.g. not-linked for a demuxer
on a few of its pads but not all).

It's better to not have these "convenience" macros but instead let people
*think* about the handling of different flow returns, that makes sense for
their element. And we should document the expected handling of flow returns for
different classes of elements in the plugin writer's guide.

Fixes bug #628014.