dictextension dict

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.

Ignore XPI files that begin with '@' to follow the conventions that
the extension manager's filter uses.

Use gContextMenu to read the selection to make the Dict extension
compatible with e10s.

Use the "window watcher" interface instead of calling window.open and
window.alert to improve security and compatibility in case some other
code overrides these functions.

Prefix the XPI files with "@" to match the Firefox filters for XPI files.

Increment the version to a "prerelease" status to prevent accidently
overwriting an official release candidate.