js: Simpler and more robust startup procedure

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.

emcclib.js

@@ -16,6 +16,23 @@
  */
 
 mergeInto(LibraryManager.library, {
+    /*
+     * void js_init_puzzle(void);
+     *
+     * Called at the start of main() to set up event handlers.
+     */
+    js_init_puzzle: function() {
+        initPuzzle();
+    },
+    /*
+     * void js_post_init(void);
+     *
+     * Called at the end of main() once the initial puzzle has been
+     * started.
+     */
+    js_post_init: function() {
+        post_init();
+    },
     /*
      * void js_debug(const char *message);
      *