dictextension dict

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.

Remove the binary property on context.xul, since it's text.

Creator a new prompter instead of using window.alert for the
catch all onError handler. This was done for consistency.

Refactor the code so that more of the logic for populating the
context menu is shared between bootstrap.js and context.js.
This also removes the DictExtensionXul object in favor of using
DictExtension exclusively.