Dynamic device discovery + support multiple ODrives

[dreusel]

This supports dynamic discovery of ODrives (ie devices connecting/disconnecting/reconnecting/rebooting while the program is running). It also adds support for interfacing with multiple ODrives simultaneously and adds a reboot button per ODrive.

This builds upon (but pretty much re-does) #9
This fixes #4

[dreusel]

Any thoughts about this @falkoschindler @rodja?

[rodja]

@falkoschindler is currently not in the office. He will have a look early next week.

[falkoschindler]

Hi @dreusel, thanks for this PR! I finally found the time for a review:

• I thought I could catch the fibre.ObjectLostError directly (without the need for checking __name__). But it doesn't seem to work, so I'll change it back to your solution. Anyway, the ODrive doesn't re-appear when rebooting. Does it work for you?
• I added a check for EmtpyInterface in update(). There are other places where the UI update can crash while rebooting the ODrive. But I guess that's not critical.
• I simplified the discovery_loop() quite a bit. I guess you wanted to preserve the order of re-appearing ODrives. But as far as I can tell, your implementation didn't work, because new elements are always added as the last dictionary item. And I wasn't sure if the algorithm would glitch if multiple ODrives appear in the same loop cycle, e.g. when connected via a USB hub. So I changed it to a more general approach.

From my point of view we can merge this pull request. When disconnecting and reconnecting ODrives, the UI is updated correctly. I hope it's still working for you as well.
And improving the reboot behavior might be something for another PR...