Channels can now be used as "animation containers" to be filtered further to obtain a set of subsidiary channels (i.e. F-Curves associated with some summary channel). The main use of this is that object and scene summary channels can now be defined without defining the filtering logic in three different places - once for channel filtering, once for drawing keyframes in action editor, and once for editing these keyframes. An indirect consequence of this, is that the "Only selected channels" option in Timeline will now result in only the keyframes for a selected bones getting shown (when enabled), instead of all keyframes for the active object. This was requested by Lee during Durian, and is something which has only become possible as a result of this commit.