Round the user picture on lock dialog.

All user dialogs (Linux, Windows, Mac, Android, etc..), or any representation of a user as an avatar in the web are round images.

Also, it shows an emblem icon to note that it is logged in as in other dialogs.

Both options can be made configurable by the user very easily, but at least (IMHO of course), I think it respects the preference of the majority of users.

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