Make all of FPrintManager's struct members private,
warn of errors using a _get_error() function, as object
creation can never fail.
The only error possible shouldn't really be an error though,
as it only means that no devices will be enumerated, but it
could tell us about newly plugged devices instead.
This will be needed to get better error messages to
users of the D-Bus API. We use glib's builtin gettext
support. No strings marked as translatable yet, we'll
need to mark those user-visible strings carefully and
review them.
With this patch, and the D-Bus activation patches posted earlier the
fprintd daemon is started automatically when needed.
We should have a way to exit it nicely when there are no users
anymore as well...
