When composing an email, display the encryption status for each address.
If the public key for a given address is found, show the "lock" sign
(encrypted) in front of it otherwise show the "unlocked" sign
(unencrypted). This status is fetched using the keymanager.

Initially, the status bar overlay was used for three purposes: indicate
whether there was a bitmask account configured, as an entry point to the
bitmask account wizard (by clicking on it), and as a watchdog that would popup
the bitmask account wizard everytime thunderbird started, if there was no
account configured.

We don't want any of these features, so this commit completelly removes the
status bar overlay. The issue indicated above (#6613) has to do with removal
of the last of these features (i.e. automated wizard popup).