Add "No asset declarations found." message

Author: fredsa

css/ui.css

@@ -546,6 +546,12 @@ div.asset_block.asset_highlight {
   background: rgba(153, 204, 153, 0.3);
   text-decoration: underline;
 }
+.asset_no_assets {
+  color: #999;
+  padding: 2em;
+  text-align: center;
+  font-style: italic;
+}
 .asset_error_msg {
   color: #ff6666;
   background-color: #330000;

src/ide/views/asseteditor.ts

@@ -438,8 +438,16 @@ export class AssetEditorView implements ProjectView, pixed.EditorContext {
         }
       });
       this.deferrednodes = [];
+      if (this.maindiv.children().length === 0) {
+        $('<div class="asset_no_assets"/>')
+          .text("No asset declarations found.")
+          .appendTo(this.maindiv);
+      }
       this.scrollToAssetFromHash();
     } else {
+      if (this.rootnodes.length > 0) {
+        this.maindiv.find('.asset_no_assets').remove();
+      }
       for (var node of this.rootnodes) {
         node.refreshRight();
       }