package.json: Use recommended field for package name.

According to [the reference][0], the `fullName` field is deprecated in favor
of the `title` field.

This patch changes this accordingly.

[0]: https://developer.mozilla.org/en-US/Add-ons/SDK/Tools/package_json

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
package.json: Add icon to the metadata file.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>