[Feature request] Full backup of database (export database / import database)

Please implement a full backup option for backuping database. Current K-9 is able to backup (export/import) only account settings. But for POP3 users (and in some cases it is actual for IMAP cases) it is required to have a possibility to make a full backup of the base and restore it.

Just a simple sample: you had bought a new phone and you want to switch K-9 from previous phone to a new one. It is required to move/copy database. But you can't do this.

Another sample: it is a just backup option for case if you had lost the data.

Just provide a function to select for which account you want to backup database (by default - all accounts), and the same with the restore option (which account data you want to restore from backup, by default - all accounts).

This is really required feature for POP3 users, who are using POP3 for example for security purposes, when messages are deleted from server after downloading them.

No files, tests, or entry points are named in the issue. Start by locating the existing account-settings export/import implementation and the database and account data it handles. Done means providing selectable full database export and restore, defaulting to all accounts, while supporting POP3 data.