Add form type selection and spinner for iframe

[MAILPOET-2743]

assets/css/src/components-form-editor/_preview.scss

@@ -13,6 +13,14 @@
   margin: 0;
   min-height: 100%;
   min-width: 100%;
+
+  .mailpoet_form_preview_type_select {
+    position: absolute;
+    top: 20px;
+    left: 40px;
+    z-index: 11;
+  }
+
   .mailpoet-modal-header {
     height: 26px;
     margin-bottom: 0;
@@ -33,12 +41,22 @@
     height: 100%;
     display: flex;
     flex-direction: column;
+
+    .mailpoet_spinner_wrapper {
+      left: 0;
+      position: absolute;
+      top: 0;
+    }
   }
 
   .mailpoet_browser_preview_container {
     height: 100%;
   }
 
+  .mailpoet_browser_preview_border {
+    position: relative;
+  }
+
   .mailpoet_form_preview_iframe {
     height: 100%;
     max-height: 100%;