lavfi: add avfilter_pad_get_type() and avfilter_pad_get_name().

This will allow making AVFilterPad opaque for the calling apps, since
those are the only two fields that can be useful to the users.

doc/APIchanges

@@ -13,6 +13,11 @@ libavutil:     2011-04-18
 
 API changes, most recent first:
 
+2012-xx-xx - xxxxxxx - lavfi 2.22.0 - avfilter.h
+  Add avfilter_pad_get_type() and avfilter_pad_get_name(). Those
+  should now be used instead of accessing AVFilterPad members
+  directly.
+
 2012-xx-xx - xxxxxxx - lavu 51.32.0 - audioconvert.h
   Add av_get_channel_layout_channel_index(), av_get_channel_name()
   and av_channel_layout_extract_channel().