Previously, we initialised all of the JavaScript event handlers as soon
at the DOM was loaded, and then called main() ourselves once the
Emscripten runtime was ready.  This was slightly dangerous since it
depended on none of those event handlers' being called before main().
In practice this was difficult because most of the elements the event
handlers were attached to were invisible, but it did limit what event
handlers could safely be used.

Now, the event handlers are initialised from main().  This makes things
work in a sufficiently conventional way that we can just let the
Emscripten run-time call main() in its usual way, rather than involving
ourselves in the minutiae of Emscripten's startup.

Using a stunt webserver which artificially introduces a 3s delay just
before the last line of the HTML output, I have reproduced a
uwer-reported loading/startup race bug:

Previously the wasm loading was started by the <script> element,
synchronously. If the wasm loading is fast, and finishes before the
HTML loading, the onRuntimeInitialized event may occur before
initPuzzles.  But initPuzzles sets up the event handler.

Fix this bug, and introduce a new comment containing an argument for
the correctness of the new approach.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>