Code review comment for lp:~igors/do-plugins/Thunderbird-fix

This fixes Thunderbird plugin. The original problem was described in https://bugs.launchpad.net/do-plugins/+bug/305778 and was fixed by the first commit on the branch.

The rest of the changes add the following:

 * Support for multiple Thunderbird emails on a single ContactItem (ContactDetailItems are introduced)
 * Importing emails from Thunderbird history in addition to Thunderbird address book
 * Using the most used email for main ContactItem (as determined by Thunderbird's PopularityIndex)

I know the plugin is marked as "Community" but people still seem to distribute it (e.g., Fedora does).