themes: MDL-19077 new $OUTPUT->header/footer to replace print_header/footer.

themes: MDL-19077 new $OUTPUT->header/footer to replace print_header/footer.
Also, part of the change from weblib.php functions to $OUTPUT-> methods.

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

This is a big change, and the result is not perfect yet. Expect some debugging output
on some pages.

The main part of these changes are that $OUTPUT->header now looks for a file
in the theme called layout.php, rather than header.html and footer.html. Also
you can have special templates for certain pages like layout-home.php. There is
fallback code for Moodle 1.9 themes, so they still work.

A few of the old arguments to print_header are no longer supported. (You get an
exception if you try to use them.) Sam H will be cleaning those up.

All the weblib functions that have been replaced with $OUTPUT-> have version in
deprecatedlib, so existing code will go on working for the foreseeable future.