Xfconf signal bindings branch

[Nick Schermer]

Brian (and others),

Just pushed a new xfconf branch with my revamp of the bindings. The
commit messages will tell most of the technical stuff, but basically
it is dropping the weak refs and completely use signals, just like the
exo bindings. It also uses 1 internal list for holding all the
bindings, which makes it easier to understand (and probably as fast as
the combination of the hash table and lists set on the objects).

The test I added segfaults because of a double free in the cache
(related to the OldCacheItem structure) maybe you can look into that?
The bindings seem to work fine in all existing Xfce applications and
it fixes all the errors and warnings (weak refs not found and hash
table crash) produced by the devel panel (part of bug #5493), which
was my goal of all this, obviously ;-).

So, Brian, please review the code.

Cheers,
Nick