Introduce QXmppAccountMigrationManager

[pasnox]

This manager allows to export and import account data to migrate account into another server.

PR check list:

• [ ] Document your code
• [ ] Add \since QXmpp 1.X, QXMPP_EXPORT
• [ ] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [ ] Update doc/doap.xml
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

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