themes: MDL-19077 change how the theme is initialised and CSS is served.

themes: MDL-19077 change how the theme is initialised and CSS is served.

This is part of http://docs.moodle.org/en/Development:Theme_engines_for_Moodle%3F

$THEME is now initialised at the same time as $OUTPUT. Old functions like
theme_setup are deprecated in favour of methods on $PAGE. There is a new
theme_config class in outputlib.php that deals with loading the theme config.php file.

CSS used to be served by themes styles.php files calling a function in weblib.php.
Now it works by each theme's styles.php file doing
$themename = basename(dirname(__FILE__));
require_once(dirname(__FILE__) . '/../../theme/styles.php');
which is less code to be copied into each theme. (Old-style styles.php files still
work thanks to some code in deprecatedlib.php.)

Admin UI for choosing a theme cleaned up.

A couple of theme-specific hard-coded hacks like $THEME->cssconstants and
$THEME->CSSEdit have been replaced by a more generic $THEME->customcssoutputfunction
hook. See examples at the end of outputlib.php

Also:
* Fix setting the theme in the URL, which seems to have been broken since 1.9.
* Fix up errors on a few pages caused by the new initialisation order.
* MDL-19097 moodle_page::set_course should not set $COURSE unless it is $PAGE.
* httpsrequired() from moodlelib.php moved to $PAGE->https_required().
* Move has_started() method to the renderer base class.
* Further fixes to display of early errors.
* Remove print_header/footer_old from weblib. I did not mean to commit them before.