Add method to QDBusServer to allow anonymous client connections.

This change adds a new method to QDBusServer to allow anonymous
connections. This is part of the DBus API and was not yet possible
to use with QDBusServer. It is set in the newConnection callback
when a new client tries to connect.
Anonymous connections are enabled by default in DBus but not allowed
by default.

[ChangeLog][QtDBus][QDBusServer] Added method to QDBusServer to allow
anonymous client connections, even if the connecting client is not
authenticated as a user.

Change-Id: I984c9e634101ecd2e67bb25c8d12bb1071836fd3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>