This commits cleans some code (settings and dependencies)
and one file included previous by error.
Also it disables all the IMS-CC export feature by default, being
possible to enable it under demand by setting (@ config.php):
$CFG->enablebackupconverters = true;
Added write_xml() helper that writes the given tree-ish structure into
the current xml writer. Improved get_contextid() so that it can use
indices for searching data. Added set_stash() and get_stash() helper
methods.
These are David's changes of Mark's code that replace the
plan/tasks/steps infrastructure with a bit simpler one. The changes will
be described in the next commit that will actually finish the
conversion.
TODO: refactor backup/converter/moodle1/stepslib.php into conversion
handlers.
* Several base_converter methods made protected when there was no obvious
reason why they should be public (subject of eventual change still).
* The conversion chain now constructed in advance before any converter
class is instantiated, using Dijkstra's algorithm.
