inline: Run format hook first

inline: Run format hook first

inline has a format hook that is an optimisation hack. Until this hook
runs, the inlined content is not present on the page. This can prevent
other format hooks, that process that content, from acting on inlined
content. In bug ##509710, we discovered this happened commonly for the
embed plugin, but it could in theory happen for many other plugins (color,
cutpaste, etc) that use format to fill in special html after sanitization.

The ordering was essentially random (hash key order). That's kinda a good
thing, because hooks should be independent of other hooks and able to run
in any order. But for things like inline, that just doesn't work.

To fix the immediate problem, let's make hooks able to be registered as
running "first". There was already the ability to make them run "last".

Now, this simple first/middle/last ordering is obviously not going to work
if a lot of things need to run first, or last, since then we'll be back to
being unable to specify ordering inside those sets. But before worrying about
that too much, and considering dependency ordering, etc, observe how few
plugins use last ordering: Exactly one needs it. And, so far, exactly one
needs first ordering. So for now, KISS.

Another implementation note: I could have sorted the plugins with
first/last/middle as the primary key, and plugin name secondary, to get a
guaranteed stable order. Instead, I chose to preserve hash order. Two
opposing things pulled me toward that decision:

1. Since has order is randomish, it will ensure that no accidental
   ordering assumptions are made.
2. Assume for a minute that ordering matters a lot more than expected.
   Drastically changing the order a particular configuration uses could
   result in a lot of subtle bugs cropping up. (I hope this assumption is
   false, partly due to #1, but can't rule it out.)