merge: refactor backends to use Account object

 * clang-format
 * rename Email private data
 * rename Mailbox private data
 * misc
 * add Account
 * mxapi: unify naming
 * mxapi: add Account
 * mxapi: add Mailbox to mbox_open
 * mxapi: refactoring
 * Notmuch: add Account
 * Notmuch: move private data
 * Notmuch: unify naming
 * Notmuch: move db to Account
 * Notmuch: separate db functions
 * Maildir: add Account
 * Maildir: unify naming
 * Maildir: move private data
 * Pop: add Account
 * Pop: unify naming
 * Pop: move data to Account
 * Compress: unify naming
 * Compress: add Account
 * Nntp: add Account
 * Nntp: unify naming
 * Nntp: add new/free/get functions
 * Mbox: add Account
 * Mbox: unify naming
 * Mbox: move data to Account
 * Imap: tidy
 * Imap: add Account
 * Imap: use Mailbox ptr
 * Imap: unify naming
 * Imap: refactor current message
 * Imap: move Account data