* Add type property to TextSource* classes
* Use type property rather than instanceof
* Expose a sentence offset value
* Use offset added to URL
* Improve fallback sentence for Anki note context
* Move fg/js/frame-ancestry-handler.js to js/comm/frame-ancestry-handler.js
* Move fg/js/frame-offset-forwarder.js to js/comm/frame-offset-forwarder.js
* Move fg/js/dom-text-scanner.js to js/dom/dom-text-scanner.js
* Move fg/js/text-source-element.js to js/dom/text-source-element.js
* Move fg/js/text-source-range.js to js/dom/text-source-range.js
* Move fg/js/float-main.js to js/display/popup-main.js
* Move fg/js/content-script-main.js to js/app/content-script-main.js
* Move fg/js/frontend.js to js/app/frontend.js
* Move fg/js/popup-factory.js to js/app/popup-factory.js
* Move fg/js/popup-proxy.js to js/app/popup-proxy.js
* Move fg/js/popup-window.js to js/app/popup-window.js
* Move fg/js/popup.js to js/app/popup.js
