Fprintd is dependent on the deprecated dbus-glib, also this doesn't provide
various features we can take advantage of, like the ones for async
authentication mechanism.
So, remove all the dbus-glib dependencies and simplify the code, but without
any further refactor, and keeping everything as it used to work, while this
will give room for further improvements in subsequent commits.
Internally, we just use dbus-codegen to generate the skeletons, and we
use the generated FprintdDBusManager with composition, while we
implement the device skeleton interface in FprintDevice, so that we
don't have to use it as a proxy, and keep being closer to what it used
to be with dbus-glib.
Fixes: #61
Use error messages to be consistent, and avoid checking for a returned
value when dbus-glib function to fetch it returned false, as it's
implicit that we had a failure.
Otherwise if didn't fail we are sure that we got the requested value.
If we get a `NoEnrolledPrints` error during delete, we don't consider it an
hard error and in such case we proceed to releasing the device, but without
clearing the previously set error first.
These utilities are generally useful beyond only testing purproses. And,
since it is desirable to have automated tests inside the tests
subdirecty, it makes sense to move them elsewhere.
