system_device_drivers should check for installedness

[Harald Sitter]

currently system_device_drivers() does not list whether a driver is installed or not, however since it needs to query apt for matching packages anyway it might be worthwhile to check whether those packages are installed or not.

in kubuntu we have a component dispatching notifications if a recommended driver is available but not in use, however to check the in-use part we need to access apt to check whether the driver is installed to begin with, this unnecessarily ties the notifying component directly to apt.