session: Emit SelectedUserChanged signal on user changes

Marco Trevisanrequested to merge
3v1n0/gdm:greeter-emit-user-changes into master

An underlying PAM module (such as the gdm-smartcard one) could change the selected user for the authentication based on a certificate but we never expose this change, even if we've an API for this.

So let's emit the signal and inform the greeter about.

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