add disabled state to umb-multiple-textbox property editor add button when readonly

src/Umbraco.Web.UI.Client/src/less/components/umb-multiple-textbox.less

@@ -44,6 +44,14 @@
     &:extend(.umb-editor-wrapper .umb-node-preview);
 }
 
+.umb-multiple-textbox .add-link:disabled {
+    &:extend(.umb-node-preview-add:disabled);
+}
+
+.umb-multiple-textbox .add-link:disabled:hover {
+    &:extend(.umb-node-preview-add:disabled:hover);
+}
+
 .umb-modal .umb-multiple-textbox .textbox-wrapper .umb-editor {
     flex: 1 1 auto;
     width: auto;

src/Umbraco.Web.UI.Client/src/views/propertyeditors/multipletextbox/multipletextbox.html

@@ -32,7 +32,7 @@
     <button type="button" class="add-link" localize="title" title="@content_addTextBox"
        ng-show="model.config.max <= 0 || model.value.length < model.config.max"
        ng-click="add($event)"
-       ng-attr-readonly="{{ readonly || undefined }}">
+       ng-disabled="readonly">
         <localize key="general_add">Add</localize>
     </button>