Track changes in Thunderbird. Starting in TB 25, IMAP string bundle now uses text identifiers instead of numeric.

Fixes luckrat/KeeFox #181
Merge pull request #185 from dlech/tb-imap-strings

Track changes in Thunderbird. Starting in TB 25, IMAP string bundle now ...

Look out for a build of KeeFox 1.2.5 in a few days...