This commit remove old cruft related to multiples libraries displayed
on the same page, and should fix code that was trying to refresh the
libraries, for example after registering a new atom.

Previously, if you only had Atom Reference fields in your form,
the resource library wouldn't show up. This code got refactored
in D7, this is a minimal fix for D6.

For full impact, this option should be set /before/ Scald Core is first
enabled. Note that there's no support for switching the option on/off,
which means that users created or updated while this option is off won't
be updated in the Scald Authors table.

The whole point of the function was to deal with PHP 5.3
compatibility, but it included a call time pass by reference
that generated a warning.

Olivier Friesse authored Mar 30, 2012 and DeFr committed Mar 30, 2012

This can vastly improve the lookup time for the providers that may
want to lookup the sid of an atom they've registered,

If a new type is registered, but the user doesn't save the context
form to associate it with a transcoder in all the contexts, then
the type might not have a transcoder set.

This was problematic in scald_unregister_atom, which tries to notifiy
all the configured transcoders of the atom unregistration. The code
now checks if a transcoder has been specified, and just skips the
context is no transcoder were set.

Previously, no check were done to ensure that a valid sid was given to
the function, which triggered bad interaction if the atom had already
been unregistered.

Let's make it completely clear that it's a global for internal
use only by naming it _scald_override.

The field base_description of a part of the primary key, which
made it not NULL-able in MySQL. The schema definition was thus
automatically changed by the engine from
  'not null' => FALSE,
to
  'not null' => TRUE,
  'default'  => '',

That's a suitable default too, all known providers already pass
a real value as the 'base_description', so let's use something
that's actually doable in MySQL.