[MERGE][1.14] simplify the content filter registration API

[Ian Clatworthy]

Bob Tanner wrote:
> 
> On Apr 6, 2009, at 9:11 AM, John Arbash Meinel wrote:
> 
>> - From me, you'll have to get sign-off from Bob to get it into 1.14.
> 
> 
> Approved for 1.14

Thanks Bob. Can you please cherrypick this into that branch?
(I'm offline most of the day and the keywords plugin has been
upgraded to use it so it works for those of us running bzr.dev.)

Ian C.