Engine(-Twitter),Frontend: avoid Close warning dialog (closes: #1093)

There are some protocols, like Twitter, in which there are no
consequences from disconnecting to the server: no unavailable
status is shown to our contacts, and no chat buffers will be
lost because all the info resides server-side. For these, there
is not really any point in warning the user about current open
chats when she intends to close the connection, so whitelist
Twitter in this way to avoid the pointless dialog.

src/Engine-Twitter/Protocols/Twitter/TwitterProtocolManager.cs

@@ -116,6 +116,12 @@ protected bool HasTokens {
             }
         }
 
+        public override bool HiddenPresenceStatus {
+            get {
+                return true;
+            }
+        }
+
         public TwitterProtocolManager(Session session) : base(session)
         {
             Trace.Call(session);

src/Engine/Protocols/IProtocolManager.cs

@@ -67,6 +67,10 @@ PresenceStatus PresenceStatus {
             get;
         }
 
+        bool HiddenPresenceStatus {
+            get;
+        }
+
         void Connect(FrontendManager frontendManager, ServerModel server);
         void Disconnect(FrontendManager frontendManager);
         void Reconnect(FrontendManager frontendManager);

src/Engine/Protocols/ProtocolManagerBase.cs

@@ -86,6 +86,12 @@ public virtual PresenceStatus PresenceStatus {
             }
         }
 
+        public virtual bool HiddenPresenceStatus {
+            get {
+                return false;
+            }
+        }
+
         public abstract string NetworkID {
             get;
         }

src/Frontend-GNOME/Views/Chats/ProtocolChatView.cs

@@ -165,7 +165,7 @@ public override void Close()
                     ownedChats++;
                 }
             }
-            if (ownedChats > 1) {
+            if (!ProtocolManager.HiddenPresenceStatus && ownedChats > 1) {
                 Gtk.MessageDialog md = new Gtk.MessageDialog(
                     Frontend.MainWindow,
                     Gtk.DialogFlags.Modal,