QXmppAccountMigrationManager: Make public the extensions getter/setter

[pasnox]

This is needed because without that change applications can not store and use app specific data. By example Kaidan export and import its roster settings which requires that all migration tasks finished before importing roster related data.

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [x] Update doc/doap.xml
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>