This is not used right now in all its full possibilities, but will make
devices hotplug support easier to implement and handle at client-side
level.
As per this we can stop doing the manual tracking of the devices.
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
