Shared serial port access between tabs

[tresf]

Currently, serial ports are exclusive to the tab that opened it.

Steps to reproduce:

1. Visit demo.qz.io
2. Click Serial tab
3. List serial ports
4. Claim a serial port
5. Open a new tab
6. Visit demo.qz.io
7. Click Serial tab
8. Claim the same serial port as Step 4.
   1. It will error saying it's busy
9. Try closing the port
   1. It will say it's not open

Since we currently offer a many-to-one relationship for other comms and listeners, I believe serial ports can be enhanced to support multiple tabs as well.

Assigning to @Vzor- because he's authored this many-to-one successfully in several other areas.

[tomfrew]

Yes please!!