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.

lib/emptyfile.php

@@ -0,0 +1,21 @@
+<?php
+
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * This is an empty file. For those times when you want something you can request
+ * to get an empty response.
+ */