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

[ghsrcgh]

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.