dictextension dict

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.

Fix problems with the "restartless installation" that prevented Dict from working on Thunderbird.

Implement support for restartless installation.