Fixes issue where connectivity issues can lead to persistent "Offline" status in UI

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

XIONControlPanel/Controllers/MainViewController.swift

@@ -206,11 +206,6 @@ class MainViewController: UIViewController, SwitchesViewControllerDelegate, Serv
     {
         _switchesController.devices = Array(multiplex.devices)
         _updateVisualization(false)
-        
-        // Update connection status too, if applicable
-        if _serverMultiplex.numServers == _serverMultiplex.numberOfConnectedServers() {
-            _updateConnectivityStatus(.connected)
-        }
     }
     
     func serverMultiplex(_ multiplex: ServerMultiplex, devicesStateChanged devices: [AnyDevice])
@@ -218,6 +213,13 @@ class MainViewController: UIViewController, SwitchesViewControllerDelegate, Serv
         _switchesController.devicesStateChanged(devices)
     }
     
+    func serverMultiplex(_ multiplex: ServerMultiplex, didReceiveAcknowledgementFromServer server: Server)
+    {
+        // Update connection status too, if applicable
+        let groupConnectionStatus = multiplex.groupConnectionStatus()
+        _updateConnectivityStatus(groupConnectionStatus)
+    }
+    
     func serverMultiplex(_ multiplex: ServerMultiplex, didEncounterError error: Error)
     {
         _updateConnectivityStatus(.error)