Define multiple targets for devices - fallback mode

[arthurlutz]

It would be nice to have pymakr be able to get a list of devices so that when one is not available it switches to the next. For example sometimes the device over serial (after a reset) will show as /dev/ttyUSB1 instead of /dev/ttyUSB0. If we could avoid going and changing the preference in the settings, that would be nice. Or maybe it's just simply a simple way in the UI to switch between devices.