[PATCH v3] client: Some cleanup and new _id versions of constructors

[spitzak at gmail.com]

The first three patches are cleanup that does not change the api. In particular
this change moves the malloc of wl_proxy outside the lock.

Patch 4 adds an "_id" version of all the constructors that takes an existing
wl_proxy, and a "_new" function to create these proxies. This is an alternative
to wl_proxy_wrapper which I think is simpler to use, though it adds to the
header files.

Patch 5 adds an "_id" version of wl_registry_bind.