Changed autosave handling for better editor performance

This changes how the editors interact with the parent page-editor
compontent, which handles auto-saving.
Instead of blasting the full editor content upon any change to that
parent compontent, the editors just alert of a change, without the
content. The parent compontent then requests the editor content from the
editor component when it needs that data for an autosave.

For #3981

resources/js/components/markdown-editor.js

@@ -137,4 +137,13 @@ export class MarkdownEditor extends Component {
         return drawioAttrEl.getAttribute('drawio-url') || '';
     }
 
+    /**
+     * Get the content of this editor.
+     * Used by the parent page editor component.
+     * @return {{html: String, markdown: String}}
+     */
+    getContent() {
+        return this.editor.actions.getContent();
+    }
+
 }