Enhanced header in Accounts, MessageList and Folder to show the unread

count, scoped for the Activity, the in-progress operation, the account
on which the operation is in progress, the folder for the operation,
when appropriate, and the progress of the operation, when it applies
to multiple items.  For the MessageList, also use the determinate
progress bar to show progress for synchronization of the folder being
displayed.

Fixes Issue 924.

Also, a minor change that might help with Issue 913, by putting the
insertion of the pending command into a background thread.