- Fix: Avoid extension includeWith button conflicts/redundancies;

Incorporates #147
- Fix: Ensure shift-key cycling through flyouts works with extension-added
    `includeWith` as well as toolbarbuttons
- Fix: Apply flyout arrows after extensions callback
- Fix: Ensure SVG icon of flyout right-arrow is cloned to can be applied to
    more than one extension
- Fix: Ensure line tool shows as selected when "L" key command is used
- Refactoring: Avoid passing on `undefined` var. (#147)
- Refactoring: lbs; avoid indent in connector, destructuring, use map over push
- Docs: Clarify nature of fixes
- Docs: JSDoc for `setupFlyouts`, `Actions`, `toggleSidePanel`; missing for
  ToolbarButton

editor/svgcanvas.js

@@ -1156,7 +1156,7 @@ this.addExtension = async function (name, extInitFunc, importLocale) {
     }
 
     extensions[name] = extObj;
-    call('extension_added', extObj);
+    return call('extension_added', extObj);
   } else {
     console.log('Cannot add extension "' + name + '", an extension by that name already exists.');
   }