themes: MDL-19077 - more work on the theme_config class.

themes: MDL-19077 - more work on the theme_config class.

* Writing lots more PHPdoc comments for all the settings.
* Strip the comments from the standard theme config.php (see below)
* Move finding the layout template from moodle_core_renderer to the theme_config class
* Move loading meta.php files from moodle_core_renderer to the theme_config class
* MDL-19719 Remove support for $THEME->langsheets, and lang/.../styles.php
* MDL-19719 Put the Vietnamese-specific code into the standard theme
* Support for styles.php in all plugin types (at the theme's discretion). (c/f MDL-16438)
* More PHP doc comments for moodle_core_renderer methods.

About stripping comments from config.php: I'm not sure if this
is a good idea, and I may revert this bit. I am hoping that once
http://phpdocs.moodle.org/HEAD/moodlecore/theme_config.html has
been updated, that will be better than copying and pasting these
comments into every theme. I intend to post in the themes forum
to canvas opinions.