* Simplifies libusb context usage and allows us to set options for
all contexts easily. Notably, this lets us enable usbdk support
in libusb, which is now opt-in in the latest version.
* Moves the libusb config descriptor wrapper class to LibusbUtils too
since that could easily be reused.
* Moves device listing to LibusbUtils too and add a lock around it
as some libusb backends are not thread safe.
* Consequences: only a single context and a single event handling
thread is used now, which is more efficient.
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