cs_thunderbolt.py: re-work module initialization, code cleanup

[timfeierabend]

Re-worked settings module initialization to simplify the startup checks. The main goal is to use D-Bus for all checks. The current solution performs 3 checks:

1. Checks for whether bolt is installed by checking if boltctl exists on the file system.
2. Checks for whether the system has thunderbolt by querying sysfs.
3. Checks if the bolt daemon is alive.

This new implementation performs 2 checks:

1. Checks if org.freedesktop.bolt is in the D-Bus activitable list. If it is installed and activatable, it will appear in this list. This replaces check number 1 above. Since this settings module uses D-Bus for all functionality, this is a more accurate check.
2. If bolt is available, we initialize the Manager, and get the list of domains. Per the documentation, there will be 1 domain/device pair with the same uuid per thunderbolt controller in the system. This replaces check number 2 above. boltd knows how to examine sysfs, no need to re-invent the wheel, lets just ask it if it sees thunderbolt support on the system.

The check for number 3 is removed because if org.freedesktop.bolt isn't running, it will automatically start when we start interacting with it. Systems that do have thunderbolt, but don't have any devices connected via thunderbolt, won't automatically start the service until a device connects.

The wrapper classes around the DBus proxy objects were re-factored to make use of get_cached_property() feature of Gio.DBusProxy objects. No need to re-invent the wheel and manage a property cache when the proxy already does this. The "Proxy" part of their names were removed.