Fix compiler warnings

Ooui/Button.cs

@@ -7,7 +7,6 @@ namespace Ooui
 {
     public class Button : FormControl
     {
-        ButtonType typ = ButtonType.Submit;
         public ButtonType Type {
             get => GetAttribute ("type", ButtonType.Submit);
             set => SetAttributeProperty ("type", value);

Ooui/Element.cs

@@ -20,7 +20,6 @@ namespace Ooui
             set => SetAttributeProperty ("title", value);
         }
 
-        bool hidden = false;
         public bool IsHidden {
             get => GetBooleanAttribute ("hidden");
             set => SetBooleanAttributeProperty ("hidden", value);

Ooui/Form.cs

@@ -4,7 +4,6 @@ namespace Ooui
 {
     public class Form : Element
     {
-        string action = "";
         public string Action {
             get => GetStringAttribute ("action", "");
             set => SetAttributeProperty ("action", value ?? "");

Ooui/FormControl.cs

@@ -9,7 +9,6 @@ namespace Ooui
             set => SetAttributeProperty ("name", value);
         }
 
-        bool isDisabled = false;
         public bool IsDisabled {
             get => GetBooleanAttribute ("disabled");
             set => SetBooleanAttributeProperty ("disabled", value);

Ooui/Ooui.csproj

@@ -9,6 +9,7 @@
     <PackageLicenseUrl>https://github.com/praeclarum/Ooui/blob/master/LICENSE</PackageLicenseUrl>
     <RepositoryUrl>https://github.com/praeclarum/Ooui.git</RepositoryUrl>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>

Ooui/TextArea.cs

@@ -25,7 +25,6 @@ namespace Ooui
             set => SetAttributeProperty ("rows", value);
         }
 
-        int cols = 20;
         public int Columns {
             get => GetAttribute ("cols", 20);
             set => SetAttributeProperty ("cols", value);

Ooui/WebSocketSession.cs

@@ -115,50 +115,6 @@ namespace Ooui
             }
         }
 
-        void QueueStateMessagesLocked (EventTarget target)
-        {
-            if (target == null) return;
-            var created = false;
-            foreach (var m in target.StateMessages) {
-                if (m.MessageType == MessageType.Create) {
-                    createdIds.Add (m.TargetId);
-                    created = true;
-                }
-                if (created) {
-                    QueueMessageLocked (m);
-                }
-            }
-        }
-
-        void QueueMessageLocked (Message message)
-        {
-            //
-            // Make sure all the referenced objects have been created
-            //
-            if (!createdIds.Contains (message.TargetId)) {
-                QueueStateMessagesLocked (element.GetElementById (message.TargetId));
-            }
-            if (message.Value is EventTarget ve) {
-                if (!createdIds.Contains (ve.Id)) {
-                    QueueStateMessagesLocked (ve);
-                }
-            }
-            else if (message.Value is Array a) {
-                for (var i = 0; i < a.Length; i++) {
-                    // Console.WriteLine ($"A{i} = {a.GetValue(i)}");
-                    if (a.GetValue (i) is EventTarget e && !createdIds.Contains (e.Id)) {
-                        QueueStateMessagesLocked (e);
-                    }
-                }
-            }
-
-            //
-            // Add it to the queue
-            //
-            //Console.WriteLine ($"QM {message.MessageType} {message.TargetId} {message.Key} {message.Value}");
-            queuedMessages.Add (message);
-        }
-
         protected override void QueueMessage (Message message)
         {
             base.QueueMessage (message);