Please add a way to create a Manager instance with a custom QDBusConnection

For my project's test suite, I use libqtdbustest to set up a private session bus in order to isolate them from my user session, and to isolate tests from each other. To work reliably, tests need to use the QDBusConnection provided by libqtdbustest rather than the shared connection returned by QDBusConnection:sessionBus().

At present there is no way to tell libOnlineAccountsQt to use an alternative connection, so it may end up trying to talk to a stale session bus that has been shut down.

Ideally, I'd like a second constructor to Manager that takes a connection. Something like:

    Manager(const QString& applicationId, const QDBusConnection& bus, QObject *parent=nullptr);

After quick look at the source code, this doesn't look like it'd be too difficult to add.