Implement model that lists users and allows creating, removing 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.
Signed-off-by: Tomi Leppänen <tomi.leppanen@jolla.com>