Removed ``Mode.getContextualMenuItems``.

It was unused.
Removed the TransformationRegistry class.
Code cleanup.

Removed unused imports + some variable renames.
Code cleanup.
Code cleanup.
Updated doc.
Got rid of unnecessary tests.

The validator module contained tests that used to be useful when the
data tree and gui tree were a single tree. These tests have been
removed.
Reduced dependency on jQuery.
Fixed a bug in how text nodes were handled.

_getWalkerAt did not handle text nodes quite correctly. This has been fixed.
Point out that it is a normalized tree.
_fireTextEventIfNeeded removed.

This function had ceased to do what it advertised quite a long time
ago. It had become largely pointless. So it has been removed. Its work
is now done differently.
Fixed a bug with _cycle_entered.
Drop the use of $.data() in validator.js.
Reduced dependency on jQuery.
Use classList to get the original name.
Reduced use of jQuery.
makeElement was not used.
API change to return Node rather than jQuery objects.
Removed dependency on jQuery from transformation.
Removed unused import.