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/Servers/HubitatServer.swift

@@ -16,7 +16,7 @@ public enum HubitatServerError : Error
 
 public class HubitatServer : Server
 {
-    var connected: Bool = false
+    var connectionStatus: ConnectionStatus = .disconnected
     
     public fileprivate(set) var devices: [HubitatDevice] = []
     
@@ -33,13 +33,13 @@ public class HubitatServer : Server
     func connect(_ completion: @escaping (Error?) -> Void)
     {
         // RESTful, assume we're connected unless we get an error
-        self.connected = true
+        connectionStatus = .connected
         completion(nil)
     }
     
     func disconnect(_ completion: (Error?) -> Void)
     {
-        self.connected = false
+        connectionStatus = .disconnected
     }
     
     func fetchDevices(_ fetchCompletion: @escaping (Result<[AnyDevice], Error>) -> Void)
@@ -52,10 +52,10 @@ public class HubitatServer : Server
             .decode(type: [HubitatDevice].self, decoder: JSONDecoder())
             .sink(receiveCompletion: { completion in
                 if case let Subscribers.Completion.failure(error) = completion {
-                    self.connected = false
+                    self.connectionStatus = .error
                     fetchCompletion(.failure(error))
                 } else {
-                    self.connected = true
+                    self.connectionStatus = .connected
                 }
             }) { (devices: [HubitatDevice]) in
                 self.devices = devices