Fixes Issue 1474

Fixes Issue 1562

Issue 1474:
Provide new facility to totally wipe all data for an account but leave
settings intact.  This is useful because sometimes storage is so full
that SQLite cannot perform the usual VACUUM or message deletion.

Add confirmation dialogs to destructive Clear and Recreate operations.

Remove destructive Clear from FolderList so as not to have to
duplicate the confirmation dialogs.

Issue 1562:
Suppress notifications when new messages arrive in Trash, Sent or
Drafts special folders.