This implements device discovery with libudev, which is part of the udev
source package (and hence available in any recent Linux distribution).
Both cold and hot plugging (w.r.t. VLC) are implemented.

TODO:
 * V4L1 devices (currently V4L2 is assumed)
 * other subsystems (linux-dvb -> DVB, block -> discs)
 * item removal on unplug
 * better item name and category for non-USB devices