Use a custom protocol when loading HTML

[just-be-dev]

Fixes #70

When I first reported tauri-apps/wry#1367, @amrbashir recommended I use a custom protocol instead of with_html. I hadn't experimented enough with custom protocols at the time and didn't real understand the tradeoffs between that an with_html. The challenge with using with_html is that the origin is null which means the page is considered to be in an insecure context. That disables a lot of features (like clipboard, noted in #70). The other challenge with with_html is that when you reload you lose the contents of the page (which is what tauri-apps/wry#1367 was all about). I'd implemented load_html in tauri-apps/wry#1368, but that really doesn't get around the other problems of with_html.

This PR implements a custom protocol called load-html://. The request itself is ignored, it's just used to trigger the protocol load. There's a RefCell in the scope of the process where the HTML is cached. When the webview is initialized with html, that initial HTML string is loaded into the ref cell and the webview is initialized with the url load-html://init. Likewise, when webview.loadHtml is called, it sets the refcell and loads load-html://load/<id>.