Add gs_usb support in Windows build to support CANable, CAN Bus Debugger CANtact devices

[BennyEvans]

As discussed in Issue #238 this PR adds support for devices utilising the gs_usb protocol on Windows.

The dll that supports the protocol is licenced under the LGPL and so is appropriately compiled separately and linked dynamically. The source for the dll is available here: https://github.com/BennyEvans/candle_dll

I've tested with CANable and CAN Bus Debugger devices and with both at the same time without issue.

For transparency sake, it's worth noting that I'm the main developer of the CAN Bus Debugger device.

Let me know if there is anything that needs to be modified. I think it'll be great to add this support to the application.

Thanks again!
Ben

[BennyEvans]

It's also worth noting that the DLL/licence will need to be copied into the appropriate place to be distributed with a Windows release.

[collin80]

Thanks, I want to merge this but I'll probably hold off until I have time to also configure the continuous build to package the DLL properly.