When managing multiple accounts it's hard to determine which one you're
currently viewing. This patch makes sidebar look like this:
+++++++++++++++++++++++++
luke@lukesmith +
----------------------- +
INBOX 99+
Sent Mail +
Drafts +
Trash +
Spam +
+++++++++++++++++++++++++
The only problem with it is the necessity of changing sidebar_format, as with
its current value these 2 excess mailboxes are being appended with 0. That
leads to situation when size of an empty mailbox is being displayed as empty
string instead of 0. I consider it's not a big deal.
On account change new account's mailboxes are being appended to the old
one's. This commit forces mailboxes list to be cleared before adding new
items to it.