Add a device-class condition to libusb connect #31
Description
Currently, it is possible to specify the vendorId and productId of the device to be proxied in DeviceProxy_LibUSB::connect.
We should add a method to select any device which has a specific device class. That way we do not need to change these values for each printer model which will be used in the future.
We want the software to recognize that a newly connected device is a printer (i.e. has a specific device class) and automatically starts using it (hot-plug approach). Other device classes should be ignored.