PlugOS refactor and other tweaks (#631)

* Prep for in-process plug loading (e.g. for CF workers, Deno Deploy)
* Prototype of fixed in-process loading plugs
* Fix: buttons not to scroll with content
* Better positioning of modal especially on mobile
* Move query caching outside query
* Fix annoying mouse behavior when filter box appears
* Page navigator search tweaks

web/cm_plugins/markdown_widget.ts

@@ -132,7 +132,7 @@ export class MarkdownWidget extends WidgetType {
       buttons.filter((button) => !button.widgetTarget).map((button, idx) =>
         `<button data-button="${idx}" title="${button.description}">${button.svg}</button> `
       ).join("")
-    }</div>${html}`;
+    }</div><div class="content">${html}</div>`;
   }
 
   private attachListeners(div: HTMLElement, buttons?: CodeWidgetButton[]) {