Sidebar Toggle doesn't load core.js

### Issue
In current staging there is a new feature which allows to hide the sidebar.
For that it uses a new JS function in /media/system/core.js` named `Joomla.toggleSidebar()`.
However the JLayouts don't load `core.js`. This works currently for most views because there are other places where it is loaded. But if your extension has a pure informational page like mine has, then it breaks the whole thing.

### Solution
Make sure core.js is loaded by calling `JHtmlBehavior::core()` in the JLayout.

### Testing
In Core this doesn't happen (afaik). So the easiest way would be to install an extension with the issue. You can use my extension "SermonSpeaker" (use Webinstaller or http://www.sermonspeaker.net/download/sermonspeaker-component/sermonspeaker-component-5-2-3/pkg_sermonspeaker-zip-3.html) and navigate to the "Help" view.