NOTE that currently this is implemented for GATT characteristics only,
but can be extended to other types of objects.
Listening for property changes in a bluez characteristic requires a
long-lived connection or proxy, so this also refactors bluez device
UUIDs into a simple object that keeps the path, the proxy and the signal
id in case a callback is registered for this UUID.
An additional advantage of this is that fwupd no longer creates a
throwaway proxy object for every read and write UUID operation.
