* Create FrameClient and FrameEndpoint
* Use new Frame* classes for Popup=>frame connection
* Update api.sendMessageToFrame and api.broadcastTab to include the sender's frameId
* Update FrameClient to store the frame's frameId
* Move "yomichan" object setup to a separate file
* Update script imports
* Align message handlers
* Rename Yomichan.prepare to Yomichan.ready
* Add new prepare function
* Improve isExtensionUrl
* Use new CrossFrameAPI for popup proxy communication
* Remove use of old cross-frame communication classes
* Remove use of old cross-frame communication files
* Make the crossFrame object a member of the api object
* Remove unnecessary load of /fg/css/client.css
* Replace dynamicLoader.loadStyles with dynamicLoader.loadStyle
* Replace Popup._injectStylesheet with dynamicLoader.loadStyle
* Remove unused global
* Update formatting
* Merge default tag color with category-less tags
* Use CSS variables for colors
* Add dark theme colors
* Move color variables into display.css
* Remove old stylesheets
* Update headings
* Bump minimum Firefox version to support CSS var()
* Revert popular kanji text color
* Rename PopupProxyHost to PopupFactory
* Update FrontendApiReceiver to support non-async handlers
* Make some functions non-async
* Make setCustomCss non-async
* Make setContentScale non-async
* Remove static
* Rename variables
* Pass frameId into PopupFactory's constructor
* Change FrontendApiReceiver source from popup-proxy-host to popup-factor
* Rename _invokeHostApi to _invoke
* Rename PopupProxy.getHostUrl to getUrl
