Improve Admin Colors CSS enqueuing

[westi]

The hacky way in which we enqueue the admin colors css means that you get different ordering of the CSS between the development version of the scripts and the script-loader sourced version of the scripts.

This can cause CSS bugs which don't show when developing but do when running with the script loader - e.g. #16827

It also makes it harder to implement a custom css concatenator.

We should do something like - http://core.trac.wordpress.org/attachment/ticket/16827/colors-hacked-fixed.diff

To make this properly enqueue the styles.

We should also make this kind of call fire a _doing_it_wrong() as true is not a url :)

$styles->add( 'colors', true, array('wp-admin') );

This code harks back from [7976]