dictextension dict

Fix recent regressions that prevented Dict from working on
recent versions of Thunderbird and Seamonkey.

Fix the wording on the closest matches page to use "look up" instead of "define",
since "define" means "ascribing a meaning to", not "acertaining the meaning of".

Remove the unreferenced "pref" module, which became unnecessary
to include in this file after some earlier refactoring.

Also fix some comments and replace a dump with console.log.

Add some defensive initialization to clean up any old context menus
before adding new ones. For some reason, a "Lookup Selection" item
got leaked during development and this was never cleaned up. The
added logic ensures that two items with the same id are not created in
the context menu.

Fix a bug where windows opened with dict:// protocols did not get resized
to fix the content.

The problem is that "onload" handlers didn't get executed from the context
of a dict protocol handler (I don't know why). My fix was to use a different,
if less appropriate, event that does get executed (onfocus).

Fix bug #10; the generated dict:// links no longer includes URI-encoded
HTML character entities for words (it now correctly merely URI-encodes
the characters).

Bug #8; Additionally register a dict:// protocol handler from a
frame script so that dict:// URLs work on e10s versions of Firefox.

Use the configuration manager to determine if a dict:// protocol
is registered as a means of determining whether or not to convert
cross-referenced words into dict:// URLs. This is more correct
than assuming that Thunderbird doesn't support dict:// and all
other browsers do.

Remove the "onLoad" event handler after it is first invoked.
This seems to be the preferred style on the MDN for this sort of event handler.

Use DictExtension instead of DictExtensionXul in context.xul, since
the DIctExtensionXul object has been removed and its reusable methods
moved to DictExtension.