This fixes the problem where either internet connectivity issues on the device or intermittant
server issues can cause a persistent "OFFLINE" indicator even though that is not the case.
This requires the server protocol to be more granular about what their connection status is,
and adds a new signal from the multiplexer to the MainViewController allowing it to notify the
delegate that an acknowledgement was heard from one of the servers, allowing the main view
controller to update its UI according to whatever status best represents the group of servers
as a whole.
Testing
- This can be tested by using the network link conditioner to simulate connection issues
*after* the control panel has connected to all servers in the multiplex.
- Tested losing internet connection and regaining internet connection
- Tested starting off with no connection and regaining connection later
- Also moves all the existing Wemo* stuff over to support the multiplexer
- Moves MainViewController to be the owner of the multiplexer, and its delegate
This switches to using the more modern collection view diffable data source API.
Also I moved the action cells into their own section so we don't need to do
arithmetic to determine whether or not a cell is an action cell or a device cell.
