Fixes bulk actions on IMAP accounts: archiving, moving or deleting a large selection (hundreds of messages) could fail with "couldn't complete the action". Each action now goes through as a single batch per folder instead of one command per message, so large selections complete, and Undo brings the whole batch back.

Adds "Select all" to the selection toolbar. It used to disappear as soon as you selected your first message, so there was no way to grow a manual selection to everything at once.
