dictextension dict

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.

Tag version 0.8.4 for a formal release.

Fix a warning found by addons.mozilla.org during the submission of 0.8.3.

Tag the 0.8.3 release.

Increment the version to 0.8.3 for another release.

Ticket #6 - Improve keyboard accessibility to the Closest Matches dialog
- Listbox is given focus by default.
- Enter on a listbox item looks up its definition.
- Esc closes the dialog.

Fix an inconsistency in the text for "looking up a definition".
Historically, this was referred to as "define", which is incorrect.