Add model to access user info
Implement model that lists users and allows creating, removing them and changing their names. Users are listed with libc operations while changing them requires user-managerd DBus daemon. Information is also updated when users are changed through user-managerd. Note that UserInfo doesn't have guarantees of keeping the user in sync unless UserModel is created.
User creation happens through placeholder element that must be used to set user's full name before calling createUser. Placeholder is always the last row of the user list (if enabled of course).
Additionally implement UserInfo class that displays user information. Default constructor creates user for currently logged in user. UserInfo doesn't update automatically by itself but the data is shared between instances and thus updating one instance of the same user will change the others.